Patent Publication Number: US-6911782-B2

Title: Field emission display with separated upper electrode structure

Description:
BACKGROUND OF THE INVENTION 
   This application claims priority from Korean Patent Application No. 2002-46175, filed on Aug. 5, 2002, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference. 
   1. Field of the Invention 
   The present invention relates to a field emission display, and more particularly, to a field emission display with separate upper electrodes. 
   2. Description of the Related Art 
   Field emission displays, like cathode ray tubes (CRTs), display a color image by emitting light of a predetermined color through the bombardment of electrons onto a field emitter array (FEA) coated with phosphor. 
   The simplest way to display color images on field emission displays is a pixel-to-pixel method or a cathode switching method. In the pixel-to-pixel method, each pixel includes phosphors of different colors arrayed on corresponding anodes. A cathode is driven to hit a phosphor of a desired color with electrons. 
     FIG. 1  is a sectional view of a conventional field emission display utilizing the switching method. In  FIG. 1 , “∘” denotes applying voltage, and “X ” denotes not applying voltage. 
   Referring to  FIG. 1 , a field emitter array (FER)  20 , including a plurality of emitters  26 R,  26 G,  26 B,  28 R,  28 G, and  28 B, is formed on a cathode  11  and faces an anode  13 . Sequences of red, green, and blue phosphors  16 R,  16 G, and  16 B and  18 R,  18 G, and  18 B are arranged on the anode  13  and aligned with the respective emitters  26 R,  26 G,  26 B,  28 R,  28 G, and  28 B. A first pixel  16  includes red, green, and blue phosphors  16 R,  16 G, and  16 B, and a second pixel  18  includes red, green, and blue phosphors  18 R,  18 G, and  18 B. For the convenience of illustration, only two pixels  16  and  18  appear in  FIG. 1   
   The anode  13  is a common electrode through which a voltage is applied to all of the red, green, and blue phosphors  16 R,  16 G,  16 B,  18 R,  18 G, and  18 B, whereas the cathode  11  is comprised of individual electrodes arranged in rows and columns, through which a voltage is selectively applied to those emitters among the emitters  26 R,  26 G,  26 B,  28 R,  28 G, and  28 B that face phosphors of desired colors. 
   According to the cathode switching method, in order to emit violet (V) light  31  through the first pixel  16 , a common voltage is applied to the anode  13 , and a voltage is applied only to the emitters  26 R and  26 B facing red phosphor  16 R and blue phosphor  16 B to simultaneously emit red light and blue light. In order to emit green light G through the second pixel  18 , a common voltage is applied to the anode  13 , a voltage is applied to operate only the emitter  28 G facing green phosphor  18 G to emit green light. The cathode switching method of selectively driving an emitter facing phosphor of a desired color is simple. 
   However, the cathode switching method may cause cross talk between different colors of light. 
   For a higher resolution field emission display, phosphors  16 R,  16 G,  16 B,  18 R,  18 G, and  18 B are spaced to be closer together, and the size of the emitters  26 R,  26 G,  26 B,  28 R,  28 G, and  28 B is reduced. When such a higher resolution field emission display is driven using the above-described cathode switching method and an equal amount of voltage is simultaneously applied to all of the red, green, and blue phosphors  16 R,  16 G,  16 B,  18 R,  18 G, and  18 B, electrons emitted from the emitter  26 R, which is for exciting red phosphor  16 R, may hit green phosphor  16 G. Such cross talk degrades color purity or quality of displayed images. 
   Such a cross-talk phenomenon is illustrated in FIG.  2 . Electrons emitted from the emitter  26 B, which is for exciting blue phosphor  16 B, may reach adjacent green phosphor  16 B or red phosphor  18 R and emit undesired green or red light. Electrons emitted from the emitter  28 G, which is for exciting green phosphor  18 G, may reach adjacent red phosphor  18 R or blue phosphor  18 B. 
   In addition to the problem of cross talk, the cathode switching method requires more, smaller emitters corresponding to each color of phosphor, so that it is difficult to manufacture and assemble such emitters in a device. 
   An anode switching method can be applied to drive a color field emission display. In the anode switching method, emitters are designed to excite phosphors of different colors in each frame, and each emitter corresponds phosphors of to the three primary colors. 
     FIG. 3  is a sectional view of a conventional field emission display utilizing an anode switching method. 
   Referring to  FIG. 3 , emitters  20   a  and  20   b  are arranged on a cathode  11  facing an upper substrate  12 . A red phosphor  16 R, a green phosphor  16 G, a blue phosphor  16 B, a red phosphor  18 R, a green phosphor  18 G, and a blue phosphor  18 B are arranged on the upper substrate  12  such that each group of red, green, and blue phosphors is aligned with a respective one of the emitters  20   a  and  20   b . A first pixel  16  includes the red, green, and blue phosphors  16 R,  16 G, and  16 B, which correspond to the emitter  20   a , and a second pixel  18  includes the red, green, and blue phosphors  18 R,  18 G, and  18 B, which correspond to the emitter  20   b . First through third anodes  13   a ,  13   b , and  13   c  are formed in the upper substrate  12 . The first anode  13   a  is connected to the red phosphor  16 R in the first pixel  16  and the red phosphor  18 R in the second pixel  18 . The second anode  13   b  is connected to the green phosphor  16 G in the first pixel  16  and the green phosphor  18 G in the second pixel  18 . The third anode  13   c  is connected to the blue phosphor  16 B in the first pixel  16  and the blue phosphor  18 B in the second pixel  18 . 
   In order to emit violet (V) light  31  through the first pixel  16 , as illustrated in (a) of  FIG. 3 , a voltage is applied to the first anode  13   a  connected to the red phosphor  16 R, and a voltage is applied to the cathode  11  to drive only the emitter  20   a  corresponding to the red phosphor  16 R. In other words, only the emitter  20   a  of the first pixel  16  is driven to emit electrons, and a voltage is applied to the first anode  13   a  to allow only the red phosphor  16 R connected to the first anode  13   a  to be excited by the bombardment of the electrons, so that red light is emitted through the first pixel  16 . 
   Next, as illustrated in (b) of  FIG. 3 , a voltage is applied to the third anode  13   c  connected to the blue phosphor  16 B, and a voltage is applied to the cathode  11  to drive only the emitter  20   a  corresponding to the blue phosphor  16 B, so as to bombard and excite only the blue phosphor  16 B with electrons emitted from the emitter  20 . As a result, blue light is emitted from the first pixel  16  a short time lag after the emission of the red light so that violet (V) light  31  is perceived from the first pixel  16 . 
   In order to emit green (G) light  33  through the second pixel  18 , as illustrated in (c) of  FIG. 3 , a voltage is applied only to the second anode  13   b  connected to the green phosphor  18 G in the second pixel  18 , and a voltage is applied to the cathode  11  to allow only the emitter  20   b  corresponding to the green phosphor  18 G to emit electrons, so that green light is emitted from green phosphor  18 G in the second pixel  13 . 
   Unlike the cathode switching method, the anode switching method involves selectively applying a voltage to an anode aligned with a phosphor of a desired color. Accordingly, emitted electrons can be more accelerated toward the phosphor. In addition, the overall manufacturing process is simplified because each emitter needs not to be arranged to be aligned with each color of phosphor. However, the anode switching method requires individual anodes to be separately insulated in order to make it possible to selectively apply a voltage to an anode to obtain a desired color. Insulating three anodes, aligned with each emitter, on a 2-dimensional plane is complicated. In addition, it is impossible to apply a high voltage to the anodes due to the inherent characteristics of insulating materials. The voltage applied to the anodes is lower than when using the cathode switching method, so that the luminance of images displayed on pixels is greatly degraded. 
   SUMMARY OF THE INVENTION 
   The present invention provides a field emission display capable of displaying quality, high-luminance images by adopting an improved upper substrate structure including two separate upper electrodes for each emitter. In the field emission display, cross talk, which occurs when a cathode switching method is applied, is prevented. In addition, the field emission display can be manufactured with more ease because fewer anodes, which are aligned with each color of phosphor, correspond to each emitter than in a conventional field emission display utilizing an anode switching method. 
   In accordance with an aspect of the present invention, there is provided a field emission display comprising: a lower substrate; lower electrodes arranged as stripes on the lower substrate; a field emitter array including a plurality of emitters arranged at a predetermined interval on each of the lower electrodes; an upper substrate which faces the lower substrate; upper electrodes arranged as stripes on the upper substrate to intersect the lower electrodes; and a phosphor array including a plurality of phosphors arranged on the upper electrodes, each phosphor pair of different colors being aligned with a respective one of the emitters, wherein an upper electrode aligned with each emitter is comprised of first and second upper electrodes connected to a respective phosphor pair of different colors. 
   According to specific embodiments of the field emitter display, the emitters may comprise: a bus electrode layer arranged on a lower electrode such that a portion of the lower electrode is exposed; electron emitter tips formed on the exposed portion of the lower electrode; a gate dielectric layer formed on the bus electrode layer and having a well that surrounds the electron emitter tips; and a gate electrode layer formed on the gate dielectric layer. The electron emitter tips may be metallic tips. Alternatively, the electron emitter tips may be formed of carbon nanotubes or a carbonaceous material. 
   The phosphor array may include a repeated pattern of a red phosphor, a green phosphor, and a blue phosphor. Two adjacent phosphors of different colors which are aligned with different emitters may be connected to the first and second upper electrodes, respectively. Alternatively, two adjacent phosphors of different colors which are aligned with different emitters are both connected to one of the first and second upper electrodes. 
   The lower electrodes are cathodes, and the upper electrodes are anodes. 
   A field emission display according to the present invention has a simple, improved upper electrode structure in which two separate upper electrodes are aligned with each emitter and are connected to a phosphor pair of different colors, so that electrons emitted from each emitter can be effectively accelerated toward a phosphor of a desired color and image quality is enhanced. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other features and advantages of the present invention will become more apparent by describing in detail exemplary embodiments thereof with reference to the attached drawings in which: 
       FIG. 1  is a sectional view of a conventional field emission display utilizing a cathode switching method; 
       FIG. 2  is a sectional view illustrating cross talk in the conventional field emission display utilizing the cathode switching method; 
       FIG. 3  is a sectional view of a conventional field emission display utilizing an anode switching method, in which (a) illustrates the emission of red light in the field emission display, (b) illustrates the emission of blue light, and (c) illustrates the emission of green light; 
       FIG. 4  is a perspective view of a field emission display according to an embodiment of the present invention; 
       FIG. 5A  is a sectional view of the field emission display of  FIG. 4  illustrating a driving method for emitting violet light through a first pixel; 
       FIG. 5B  is a sectional view of the field emission display of  FIG. 4  illustrating a driving method for emitting green light through a second pixel; 
       FIG. 6  is a plan view illustrating the arrangement of upper electrodes and phosphors in the field emission display of  FIG. 4 ; 
       FIG. 7  is a sectional view illustrating cross talk in the field emission display of  FIG. 4 ; 
       FIG. 8  is a sectional view of a field emission display according to another embodiment of the present invention; 
       FIG. 9  is a plan view illustrating the arrangement of upper electrodes and phosphors in the field emission display of  FIG. 8 ; 
       FIG. 10A  is a photograph of a light emission range of a conventional field emission display utilizing a cathode switching method; 
       FIG. 10B  illustrates the spreading of electrons toward anodes in the conventional field emission display of  FIG. 10A ; and 
       FIG. 11  illustrates the result of operating the field emission display of  FIG. 8  according to the present invention under the same conditions as the conventional emission display of FIG.  10 B. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Embodiments of a field emission display according to the present invention will be described in detail with reference to the appended drawings. Identical reference numerals have been used, where possible, to designate identical elements that are commonly to the drawings. In drawings, “∘” denotes applying voltage, and “X” denotes not applying voltage. 
   Referring to  FIG. 4 , a field emission display according to an embodiment of the present invention includes a lower substrate  50  and an opposing upper substrate  52 . Cathodes (or lower electrodes)  51  are arranged as stripes on a top surface of the lower substrate  50 , and emitters  56 ,  57 , and  58  are arranged on the surface of each of the cathodes  51  at a predetermined interval. Phosphor pairs of different colors, i.e., a pair comprising a red phosphor  46 R and a green phosphor  46 G, a pair comprising a blue phosphor  47 B and a red phosphor  47 R, and a pair comprising a green phosphor  48 G and a blue phosphor  48 B, are arranged on the upper substrate  52  such that each pair is aligned with a respective one of the emitters  56 ,  57 , and  58 . Pairs of first and second anodes  53   a  and  53   b  are arranged between the upper substrate  52  and the phosphors  46 R,  46 G,  47 B,  47 R,  48 G, and  48 B such that each anode is aligned with a respective one of the phosphors. Each pair of first and second anodes  53   a  and  53   b  is connected to a phosphor pair of different colors, for example, a pair comprising a red phosphor  46 R and a green phosphor  46 G, and each phosphor pair is aligned with to an emitter, for example, the first emitter  56 . The upper substrate  52  and the lower substrate  50  are separated by a spacer  45 . 
     FIG. 5A  is a sectional view of the field emission display of FIG.  4 . The lower substrate (not shown), the cathodes  51 , only one of which is shown in  FIG. 5A , formed as stripes on the lower substrate, and a field emitter array, which includes the plurality of emitters  56 ,  57 , and  58  formed on each of the cathodes  51 , form a lower structure of the field emission display. 
   The upper substrate  52 , which faces the lower substrate  50 , the first and second anodes  53   a  and  53   b  arranged on the upper substrate  52  perpendicular to the cathodes  51 , and a phosphor array, which include multiple phosphor pairs of different colors, i.e., a pair comprising red and green phosphors  46 R,  46 G, a pair comprising blue and red phosphors  47 B and  47 R, and a pair comprising green and blue phosphors  48 G and  48 B, aligned with each of the emitters  56 ,  57 , and  58 , form an upper structure of the field emission display. 
   As described above, the first and second anodes  53   a  and  53   b  are arranged between the upper substrate  52  and the phosphor array, are aligned with each of the emitters  56 ,  57 , and  58 , and are connected with each phosphor pair of different colors, i.e., a pair of red and green phosphors  46 R and  46 G, a pair of blue and red phosphors  47 B and  47 G, and a pair of green and blue phosphors  48 G and  48 B. The red phosphor  46 R and the green phosphor  46 G aligned with the emitter  56  are connected to the first anode  53   a  and the second anode  53   b , respectively. Two adjacent phosphors which are aligned with different emitters are connected to the first and second anodes  53   a  and  53   b , respectively. For example, the red phosphor  46 G and the blue phosphor  47 B, which are aligned with the emitters  56  and  57 , are connected to the second anode  53   b  and the fist anode  53   a , respectively. 
   The first pixel  46  includes the red phosphor  46 R, the green phosphor  46 G, and the blue phosphor  46 B, the emitter  56 , and a portion of the emitter  57 . The second pixel  48  includes the red phosphor  48 R, the green phosphor  48 G, and the blue phosphor  48 B, the emitter  57 , and a portion of the emitter  58 . 
   In order to emit violet (V) light  41  through the first pixel  46  and green (G) light  43  through the second pixel  48 , as illustrated in  FIG. 5A , a voltage is applied to the first anode  53   a , which is connected with the red phosphor  46 R and the blue phosphor  47 B of the first pixel  46  and with the green phosphor  48 G of the second pixel  48 , and a voltage is applied to the emitters  56 ,  57 , and  58 , which are aligned with the red phosphor  46 R, the blue phosphor  47 B, and the green phosphor  48 G. Electrons are emitted from the emitters  56 ,  57 , and  58  and bombard the red, blue, and green phosphors  46 R,  47 B, and  48 G by electric fields created between the first anodes  53   a  and the cathode  51 , so that red light, blue light, and green light are emitted through the first and second pixels  46  and  48 . The red light and the blue light emitted through the first pixel  46  are perceived violet  41 , and the green light emitted through the second pixel  43  is perceived just green  43 . 
     FIG. 5B  illustrates a driving method for emitting red light through the second pixel  43  of the field emission display according to the present invention. 
   In order to emit red (R) light  45  through the second pixel  43 , a voltage is applied to the second anode  53   b , which is connected with the red phosphor  47 R of the second pixel  43 , and the cathode  52  is driven such that a voltage is applied only to the emitter  57  aligned with the red phosphor  47 R. As shown in  FIG. 5B , electrons are emitted from the emitter  57  and bombard the red phosphor  47 R so that the red (R) light  45  is emitted and perceived through the second pixel  43 . 
   In the above-described field emission display according to the present invention, an anode that is connected with a phosphor of a desired color in each pixel is selectively driven, and the cathode corresponding to the phosphor of a desired color is driven, so that a full range of colors can be displayed. 
   In the above field emission display according to the present invention, an upper electrode (anode) aligned with each emitter, arranged on each lower electrode (cathode), is comprised of first and second upper electrodes, which are aligned with phosphors of different colors. Therefore, the anodes and cathodes of the field emission display can be operated more easily and efficiently with this upper electrode structure. 
     FIG. 6  is a plan view illustrating the arrangement of electrodes (anodes) on the upper substrate of the field emission display according to the present invention. Phosphors  46 R,  46 G,  48 B,  47 R,  48 G, and  48 B are arranged as stripes on the upper substrate  52 , wherein each phosphor pair of different colors, i.e., a pair of red and green phosphors  46 R and  46 G, a pair of blue and red phosphors  48 B and  47 R, and a pair of green and blue phosphors  48 G and  48 B, is aligned with a respective one of the emitter  56 ,  57 , and  58 . One phosphor of each phosphor pair is connected to the first anode  53   a  and the other is connected to the second anode  53   b . In the field emission display according to the present invention, an upper electrode aligned with each emitter is comprised of only two anodes, first and second anodes  53   a  and  53   b , which are connected to each phosphor pair of different colors, so that it is easier to manufacture the upper electrode than conventional anode switching type field emission displays. 
     FIG. 7  is a sectional view illustrating a cross-talk phenomenon in the field emission display according to the present invention. When the emitter  57  is activated to emit red light through the second pixel  48 , electrons emitted from the emitter  57  may be attracted to the green phosphor  46 G of the first pixel  46 , which is aligned with the emitter  56  adjacent to the emitter  57 , as well as to the red phosphor  47 R of the second pixel  48 . 
     FIG. 8  is a sectional view of a field emission display according to another embodiment of the present invention, in which the arrangement of the first and second anodes  53  and  53   b  is varied to prevent the cross talk phenomenon in the field emission display of FIG.  7 . 
   In the field emission display of  FIG. 8 , the first and second anodes  53  and  53   b  aligned with each of the emitters  56 ,  57 ,  58 ,  76 ,  77 , and  78  and are connected to each phosphor pair of different colors, i.e., a pair of red and green phosphors  46 R and  46 G, a pair of blue and red phosphors  47 B and  47 R, a pair of green and blue phosphors  48 G and  48 B, a pair of red and green phosphors  66 R and  66 G, a pair of blue and red phosphors  67 B and  67 R, and a pair of green and blue phosphors  68 G and  68 B. Unlike the above embodiment illustrated in  FIG. 7 , two adjacent phosphors which are aligned with different emitters, for example, the green and blue phosphors  46 G and  47 B, which are aligned with the emitters  56  and  57 , the red and green phosphors  47 R and  48 G, which are aligned with the emitters  57  and  58 , the blue and red phosphors  48 B and  66 R, which are aligned with the emitters  58  and  76 , the green and blue phosphors  66 G and  67 B, which are aligned with the emitters  76  and  77 , and the red and green phosphors  67 R and  68 G, which are aligned with the emitters  77  and  78 , are connected to the same anode, i.e., the first anode  53   a  or the second anode  53   b . As a result, the spreading of electrons in the field emission display of  FIG. 7  is prevented. In  FIG. 7 , the first pixel  46  includes the red, green, and blue phosphors  46 R,  46 G, and  47 B, the second pixel  48  includes the red, green, and blue phosphors  47 R,  48 B, and  48 B, the third pixel  66  includes the red, green, and blue phosphors  66 R,  66 G, and  67 B, and the fourth pixel  68  includes the red, green, and blue phosphors  67 R,  68 G, and  68 B. 
   Red light can be emitted through the first pixel  46  by applying a voltage to the first anode  53   a  connected to the red phosphor  46 R and activating the corresponding emitter  56 . In this state, when the emitter  77  is activated, blue light can be emitted through the third pixel  66 . 
   In the field emission display of  FIG. 8  according to the present invention, light of a desired color can be emitted through each pixel by activating anodes connected to corresponding phosphors of desired colors and corresponding cathodes. 
     FIG. 9  is a plan view of the upper substrate of the field emission display of FIG.  8 . In  FIG. 9 , on the upper substrate  52 , the two adjacent phosphors  46 G and  47 B, which are aligned with the different emitters  56  and  57  in  FIG. 8 , are both connected to the second anode  53   b , and the two adjacent phosphors  47 R and  48 G, which are aligned with the different emitters  57  and  58  in  FIG. 8 , are both connected to the first anode  53   a . The upper electrode structure of the field emission display of  FIG. 9  can prevent spreading of electrons because two adjacent phosphors which are aligned with different emitters are connected to the same anode, unlike the upper electrode structure of the field emission display of FIG.  6 . 
     FIG. 10A  is a photograph of a light emission range of a conventional field emission display utilizing a cathode switching method.  FIG. 10B  illustrates the spreading of electrons toward anodes in the conventional field emission display of FIG.  10 A. 
     FIG. 11  illustrates the result of operating the field emission display of  FIG. 8  according to the present invention under the same conditions as the conventional emission display of FIG.  10 B. As is apparent from  FIG. 11 , electrons emitted from an emitter are accelerated straight toward a corresponding phosphor. 
   In a field emission display according to the present invention, two phosphors of different colors are aligned with each emitter, and the two phosphors are connected to separate upper electrodes. Such an upper electrode structure can be achieved through simpler processes compared to conventional field emission displays. In addition, the cross-talk phenomenon is prevented and image quality is improved. 
   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 details may be made therein without departing from the spirit and scope of the present invention as defined by the following claims.