Abstract:
An electron emission display, including an electron emission unit on a first substrate, a light emission unit on a second substrate, the second substrate affixed to the first substrate and having the electron emission unit and the light emission unit positioned therebetween, and a plurality of spacers disposed between the first and second substrates, wherein each spacer of the plurality of spacers includes a spacer body and at least one coating layer disposed on the spacer body, and wherein each spacer of the plurality of spacers satisfies the proviso that 0.02&lt;ρ2/ρ1&lt;100, where ρ1 is a specific resistivity of an outer-most coating layer disposed on the spacer body and ρ2 is a specific resistivity of an element in direct contact with the outer-most coating layer.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an electron emission display. In particular, the present invention relates to an electron emission display having an improved structure of spacers therebetween. 
     2. Description of the Related Art 
     In general, electron emission displays refer to devices capable of displaying images by extracting and accelerating electrons from a cathode electrode, hot or cold, toward phosphorescent layers in a vacuum environment. 
     Electron emission displays employing cold cathodes refer to devices having cathode electrodes that, instead of employing heat, emit electrons by application of a strong electric field between cathode and gate electrodes. In particular, electrons may be extracted from electron emission regions located in the cathode electrode and accelerated toward phosphorescent layers, thereby exciting the phosphorescent layers to emit visible light upon contact therebetween. 
     A conventional electron emission display may include an electron emission unit with electron emission elements, e.g., Field Emission Array (FEA), Surface Conduction Emission (SCE), Metal-Insulator-Metal (MIM), and Metal-Insulator-Semiconductor (MIS), on a first substrate, a light emission unit with phosphorescent layers on a second substrate, and a sealing member connecting the first and second substrates, such that the electron emission unit and light emission unit are enclosed in a vacuum environment, i.e., about 10 −6  torr, between the first and second substrates. 
     The vacuum environment in the electron emission display may provide high compression therein due to the large pressure difference between the interior and the exterior thereof. Accordingly, a conventional electron emission display may also include a plurality of spacers coupled between the first and second substrates to support the structure thereof. The conventional spacers may be formed of a dielectric material, e.g., glass or ceramic, to minimize a potential for a short circuit between the cathode and gate electrodes on the first substrate and the anode electrode on the second substrate. 
     However, some electrons emitted during operation of the conventional electron emission display may collide with the conventional spacers, and, consequently, charge them with a positive or negative potential with respect to the material characteristic thereof. The charged spacers may alter the electric field in the electron emission display and, thereby, modify the trajectories of the electron beams. The modified trajectories may distort the correct color expressions and quality of the electron emission display. 
     Accordingly, there exists a need to improve the structure of the spacers in the electron emission display in order to minimize color and quality distortion therein. 
     SUMMARY OF THE INVENTION 
     The present invention is therefore directed to an electron emission display, which substantially overcomes one or more of the disadvantages of the related art. 
     It is therefore a feature of an embodiment of the present invention to provide an electron emission display having improved spacers structure capable of providing enhanced color expression and quality. 
     At least one of the above and other features and advantages of the present invention may be realized by providing an electron emission display, including an electron emission unit on a first substrate, a light emission unit on a second substrate, wherein the second substrate maybe affixed to the first substrate such that the electron emission unit and the light emission unit may be positioned therebetween, and a plurality of spacers disposed between the first and second substrates, wherein each spacer of the plurality of spacers may include a spacer body and at least one coating layer disposed on the spacer body, and wherein each spacer of the plurality of spacers may satisfy the proviso that 0.02&lt;ρ2/ρ1&lt;100, where ρ1 may be a specific resistivity of an outer-most coating layer disposed on the spacer body and ρ2 may be a specific resistivity of an element in direct contact with the outer-most coating layer. The outer-most coating layer may have a secondary electron emission coefficient of about 1. 
     The at least one coating layer may be applied continuously to the spacer body. Additionally, the at least one coating layer may be formed of a metallic oxide or a carbonaceous material. The spacer body may be formed of a dielectric material. 
     Each spacer of the plurality of spacers may include a first coating layer and a second coating layer. The first coating layer may be between the body spacer and the second coating layer. The second coating layer may have a higher specific resistivity as compared to a specific resistivity of the spacer body and the first coating layer. The first coating layer may include any one of amorphous silicon doped with p-type impurities, amorphous silicon doped with n-type impurities, silicon nitride, or silicon carbide. 
     The electron emission display may further include a plurality of conductive layers, each conductive layer in communication with a respective spacer of the plurality of spacers. The plurality of conductive layers may be parallel to the first and second substrates. The electron emission display may include a plurality of conductive layers in communication with each spacer, wherein each conductive layer may be positioned between each spacer and the electron emission unit or between each spacer and the light emission unit. 
     The electron emission unit may include a plurality of electron emission regions and a plurality of driving electrodes, wherein the at least one conductive layer may be electrically connected to the plurality of driving electrodes. 
     The light emission unit may include a plurality of phosphor layers and an anode electrode, wherein the at least one conductive layer may be electrically connected to the anode electrode. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other features and advantages of the present invention will become more apparent to those of ordinary skill in the art by describing in detail exemplary embodiments thereof with reference to the attached drawings, in which: 
         FIG. 1  illustrates a schematic partial sectional view of an electron emission display according to an embodiment of the present invention; 
         FIG. 2  illustrates a schematic partial sectional view of an electron emission display according to another embodiment of the present invention; 
         FIG. 3  illustrates a schematic partial sectional view of an electron emission display according to another embodiment of the present invention; 
         FIG. 4  illustrates a partial exploded perspective view of a field emission array type electron emission display according to an embodiment of the present invention; 
         FIG. 5  illustrates a partial sectional view of a field emission array type electron emission display illustrated in  FIG. 4 ; 
         FIGS. 6A through 6D  illustrate enlarged photographs of light emission states of respective electron emission displays with spacers according to embodiments of the resent invention; and 
         FIGS. 7A through 7E  illustrate enlarged photographs of light emission states of respective electron emission displays with conventional spacers. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Korean Patent Application No. 10-2005-0101168 filed on Oct. 26, 2005, in the Korean Intellectual Property Office, and entitled: “Electron Emission Display with Spacer,” is incorporated by reference herein in its entirety. 
     The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which exemplary embodiments of the invention are illustrated. The invention may, however, be embodied in different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. 
     It will further be understood that when an element is referred to as being “on” another element or substrate, it can be directly on the other element or substrate, or intervening elements may also be present. Further, it will be understood that when an element is referred to as being “under” another element, it can be directly under, or one or more intervening elements may also be present. In addition, it will also be understood that when an element is referred to as being “between” two elements, it can be the only element between the two elements, or one or more intervening elements may also be present. Like reference numerals refer to like elements throughout. 
     An exemplary embodiment of an electron emission display according to the present invention is more fully described below with reference to  FIG. 1 . As illustrated in  FIG. 1 , an electron emission display  100 A according to an embodiment of the present invention may include an electron emission unit  14  on a first substrate  10 , a light emission unit  16  on a second substrate  12 , a sealing member (not shown) to attach the first and second substrates  10  and  12  with the electron emission and light emission units  14  and  16 , respectively, therebetween, such that a predetermined, pressure-controlled space may be formed therein, i.e., vacuum environment having pressure of about 10 −6  torr, and a plurality of spacers  28 . The first and second substrates  10  and  12  may be parallel to one another, and the electron emission unit  14  and the light emission unit  16  may face one another. 
     The electron emission unit  14  of the electron emission device  100 A according to an embodiment of the preset invention may include a plurality of electron emission regions  18  and a plurality of driving electrodes  20  for controlling the amount of electrons emitted from the electron emission regions  18 . In this respect, it should be noted that for explanatory convenience only, the driving electrodes  20  are schematically illustrated in  FIG. 1  as one electrode layer. 
     The light emission unit  16  of the electron emission device  100 A according to an embodiment of the preset invention may include a plurality of phosphor layers  22 , a plurality of black layers  24 , and an anode electrode  26 . 
     The plurality of phosphor layers  22  may be disposed on a surface of the second substrate  12  and formed of any known phosphorescent material emitting red, green and blue light. The plurality of black layers  24  may be formed on the surface of the second substrate  12  adjacent to the phosphor layers  22  to enhance the contrast of the screen. For example, the plurality of black layers  24  may be formed between phosphor layers  22 , such that each black layer  24  may be in direct communication with the second substrate  12  and two phosphor layers  22 . The anode electrode  26  may be formed on the plurality of phosphor and black layers  22  and  24  in parallel thereto, such that the plurality of phosphor and black layers  22  and  24  may be positioned between the second substrate  12  and the anode electrode  26 . 
     The anode electrode  26  may receive high voltage and, thereby, facilitate acceleration of electron beams from the first substrate  10  to the second substrate  12  and generate visible light in the phosphor layers  22  to further increase screen luminance of the electron emission display  100 A. The anode electrode  26  may be formed of any known conductive material as determined by one of ordinary skill in the art, e.g., aluminum. 
     The plurality of spacers  28  of the electron emission device  100 A according to an embodiment of the preset invention may be disposed between the first and second substrates  10  and  12  to support the structure of the electron emission display  100 A, i.e., prevent structure collapse resulting from compression formed inside the electron emission display due to pressure difference with the exterior, and to maintain the predetermined distance between the first and second substrates  10  and  12 . Each spacer  28  may be positioned to correspond to a respective black layer  24 , as illustrated in  FIG. 1 . In other words, a contact plane between each spacer  28  and the light emission unit  16  may be within a width of a respective black layer  24  in order to prevent any overlap between the spacer  28  and the phosphor layers  22  and, thereby, minimize interference with light emission from the plurality of phosphor layers  22 . 
     Each spacer of the plurality of spacers  28  may include a spacer body  281  and a coating layer  282 . The spacer body  281  may be formed of any known dielectric material, e.g., glass, ceramic, reinforced glass, photosensitive glass, and so forth, in any convenient shape, such as a bar, a pillar, and so forth. The coating layer  282  may be continuously applied to the spacer body  281  at a thickness of about 200 to about 1,000 angstroms. In this respect, it should be not that “continuous application” or like terminology refers to application of the coating layer  282  to the spacer body  281 , such that the coating layer  282  may cover the entire surface area of the spacer body  281  that may be in communication with the vacuum environment inside the electron emission display  100 A. In other words, the coating layer  282  may provide a barrier layer to the spacer body  281 , such that the surface of the spacer body  281  may not be exposed to the vacuum atmosphere. 
     The coating layer  282  may have electrical characteristics that are different from the electrical characteristics of the spacer body  281 . For example, the coating layer  282  may be formed of a metallic oxide, such as chromium oxide, or a carbonaceous material, such as diamond-like carbon, thereby exhibiting a predetermined specific resistivity that is different than the specific resistivity of the spacer body  281 . As such, the electrical characteristics of the coating layer  282  may affect and modify the overall electrical characteristics of the spacer  28 . 
     More specifically, the specific resistivity of the coating layer  282  may be lower as compared to a specific resistivity of the spacer body  281 . In this case and without intending to be bound by theory, it is believed that the coating layer  282  may function as a passage for the electrons. In other words, if electrons collide against the surface of a spacer  28 , the coating layer  282  may redirect the colliding electrons toward the electron emission unit  14  or the light emission unit  16 , thereby preventing contact between the spacer body  281  and the electrons, and, subsequently, avoiding surface-charging of the spacer  28 . 
     Alternatively, the specific resistivity of the coating layer  282  may be higher as compared to the specific resistivity of the spacer body  281 . In this case and without intending to be bound by theory, it is believed that the spacer body  281 , as opposed to the coating layer  282 , may function as a passage for the electrons. Accordingly, the coating layer  282  may be formed to have a secondary electron emission coefficient of about 1, thereby preventing the spacers  28  from being surface-charged more effectively. In this respect, it should be noted that a “secondary electron emission coefficient” refers to the number of electrons emitted from a surface per electron incident on the surface. 
     The electron emission display  100 A according to an embodiment of the present invention may be driven by application of a predetermined voltage to the plurality of driving electrodes and anode electrode  26 . More specifically, the anode electrode  26  may receive voltage that is several hundreds to several thousands volts higher than the voltage applied to the electrodes in the electron emission unit  14 , thereby providing gradual voltage elevation in the vacuum environment between the first and second substrates  10  and  12 . Accordingly, upon emission of a predetermined amount of electrons from the electron emission regions  18  into the predetermined space between the electron emission unit  14  and the light emission unit  16 , the anode electrode  26  may accelerate the electrons toward the phosphor layers  22  due to its significantly higher voltage. The accelerated electrons may collide against the phosphor layers  22  to emit light and form images. 
     Upon emission from the electron emission regions  18  and acceleration toward the phosphor layers  22 , some of the emitted electrons may collide with the spacers  28 . However, as described previously and without intending to be bound by theory, it is believed that the structure of the spacers  28  according to an embodiment of the present invention is advantageous because it may prevent surface charging of the spacers  28  due to collision with electrons. 
     According to another embodiment of the present invention illustrated in  FIG. 2 , an electron emission display  100 B may be similar to the electron emission display  100 A described with reference to  FIG. 1 , with the exception that the electron emission display  100 B may include a plurality of spacers  28 ′. 
     The plurality of spacers  28 ′ of the electron emission device  100 B according to an embodiment of the preset invention may be disposed between the first and second substrates  10  and  12  and correspond to the black layers  24 . Additionally, each spacer of the plurality of spacers  28 ′ may include a spacer body  281 , a first coating layer  283  formed on the spacer body  281 , and a second coating layer  284  formed on the first coating layer  283 . 
     The first coating layer  283  may be applied to the spacer body  281 , such that the first coating layer  283  may cover all the surface area of the spacer body  281  that may be in communication with the vacuum environment inside the electron emission display  100 B. Additionally, the first coating layer  283  may be formed of amorphous silicon doped with p-type or n-type impurities, silicon nitride (SiN), silicon carbide (SiC), or any other suitable material known in the art. 
     The second coating layer  284  may be applied to the first coating layer  283 . Additionally, the second coating layer  284  may be formed of a material identical to that of the coating layer  282  employed in the embodiment described with reference to  FIG. 1 . The second coating layer  284  may have a specific resistivity that is higher than the specific resistivity of the body  281  and the first coating layer  283 . Accordingly, and without intending to be bound by theory, it is believed that surface-charging of the spacers  28 ′ upon electron collision therewith may be minimized more efficiently. 
     According to another embodiment of the present invention illustrated in  FIG. 3 , an electron emission display  100 C may be similar to the electron emission displays  100 A and  100 B described with reference to  FIGS. 1-2 , with the exception that the electron emission display  100 C may include a plurality of conductive layers  286 . 
     For illustration convenience, it should be noted that the plurality of spacers in the electron emission display  100 C will be referred to as spacers  28 , i.e., spacers having a structure identical to the structure of the spacers  28  described previously with respect to  FIG. 1 . However, other spacers embodiments, e.g., spacers  28 ′ described previously with respect to  FIG. 2 , are not excluded from the scope of the embodiment described with respect to  FIG. 3 . 
     The plurality of spacers  28  of the electron emission device  100 C according to an embodiment of the preset invention may be disposed between the first and second substrates  10  and  12  and correspond to the black layers  24 . Additionally, each spacer of the plurality of spacers  28  may have a structure identical to the structure of the spacers  28  or the spacers  28 ′ described previously with respect to  FIGS. 1-2 , respectively. 
     At least one conductive layer  286  may be formed between each spacer  28  and either the light emission unit  16 , e.g., between the spacer  28  and the anode  26 , or the electron emission unit  14 , e.g., between the spacer  28  and the driving electrodes  20 . Accordingly, the at least one conductive layer  286  may be in electrical communication with either the driving electrode  20  or the anode electrode  26 . Alternatively, two conductive layers  286  may be disposed on each spacer  28 , such that a conductive layer  286  may be applied between each spacer  28  and both the light emission unit  16  and the electron emission unit  14 , as illustrated in  FIG. 3 . In this case, the conductive layer  286  may be in electrical communication with the driving electrode  20  and the anode electrode  26 . 
     The at least one conductive layer  286  may be in communication with the spacer body  281  and the coating layer  282 . 
     Without intending to be bound by theory, it is believed that the conductive layer  286  may lower the contact resistance between the spacer  28  and the driving electrode  20  and/or the anode electrode  26 , thereby facilitating the redirection of the colliding electron flow. 
     The spacers  28  and  28 ′ of the embodiments described above with reference to  FIG. 1-3  may be formed to satisfy the condition of formula 1 below.
 
0.02&lt;ρ2/ρ1&lt;100  Formula 1
 
where ρ1 indicates a specific resistivity of an outer-most coating layer and ρ2 indicates a specific resistivity of an element in direct contact with the outer-most coating layer. In this respect, it should be noted that an “outer-most coating layer” refers to a coating layer disposed on the spacer body  281  and being in direct contact with the vacuum environment inside the electron emission display of the present invention. Further, “an element in direct contact with the outer-most coating layer” refers to the spacer body  281  or any other layer disposed between the outer-most coating layer and the spacer body  281 . For example, with respect to the embodiment described with reference to  FIG. 1 , ρ1 indicates the specific resistivity of the coating layer  282 , and ρ2 indicates the specific resistivity of the spacer body  281 . Similarly, with respect to the embodiment described with reference to  FIG. 2 , ρ1 indicates the specific resistivity of the second coating layer  284 , and ρ2 indicates the specific resistivity of the first coating layer  283 .
 
     Without intending to be bound by theory, it is believed that redirecting colliding electrons away from spacers  28  and  28 ′ may minimize surface charge thereof, thereby reducing beam distortion, and, subsequently, suppressing secondary light emission caused by the electric charge of the spacers  28  and  28 ′. Suppression of secondary light emission may minimize visibility problems triggered by showing of the spacer on the screen, thereby improving color and picture quality of the electron emission display according to the present invention. 
     The electron emission displays  100 A,  100 B and  100 C according to the embodiments described with respect to  FIGS. 1-3  may be formed with different types of electron emission elements. For example, a FEA type electron emission display will be described in more detail with reference to  FIGS. 4-5 . In this respect, it should be noted that even though in the following exemplary embodiment of an electron emission display only FEA elements are described, other types of electron emission elements, e.g., SCE, MIM, or MIS, are not excluded from the scope of the present invention. 
     As illustrated in  FIGS. 4-5 , an FEA type electron emission display may include an electron emission unit  14 ′, a light emission unit  16 , and at least one spacer  28 . 
     The electron emission unit  14 ′ may include a plurality of electron emission regions  18 ′, a plurality of parallel first electrodes  32 , and a plurality of parallel second electrodes  34 . The plurality of first and second electrodes  32  and  34  may be positioned perpendicularly to one another with the first insulating layer  30  disposed therebetween, such that each intersection region between the first and second electrode  32  and  34  may be referred to as a pixel unit. 
     The plurality of electron emission regions  18 ′ may be electrically connected to any one of the first and the second electrodes  32  and  34 . More specifically, the electron emission regions  18 ′ may be formed in the plurality of first electrodes  32 , thereby setting the plurality of first electrodes  32  as cathode electrodes for supplying electric currents to the electron emission regions  18 ′. Accordingly, the plurality of second electrodes  34  may be set as gate electrodes for establishing voltage difference with respect to the cathode electrodes, thereby forming an electric field for inducing electron emission from the electron emission regions  18 ′. Alternatively, the electron emission regions  18 ′ may be formed in the plurality of first electrodes  32  in the plurality of second electrodes  34 , thereby setting the second electrodes  34  as cathode electrodes and the first electrodes  32  as gate electrodes. 
     The emission regions  18 ′ may be formed of any material having a low work function or a large aspect ratio and is capable of emitting electrons upon application of electric field thereto in a vacuum environment, e.g., carbonaceous material, nanometer-sized material, and so forth. For example, the electron emission regions  18 ′ may be formed of carbon nanotubes, graphite, graphite nanofibers, diamonds, diamond-like carbon, C 60 , silicon nanowires, a molybdenum-based material, a silicon-based material, or a combination thereof. If the electron emission regions  18 ′ are formed of a molybdenum-based material or a silicon-based material, the electron emission regions  18 ′ may be formed to have a pointed-tip structure. 
     As illustrated in  FIG. 4 , the electron emission unit  14 ′ of the electron emission display according to an embodiment of the present invention may further include at least one first opening  301  and at least one second opening  341  that may be formed through the first insulating layer  30  and the second electrode  34 , respectively, to expose the electron emission regions  18 ′ formed on the first electrodes  32 , such that emitted electrons may move from the electron emission regions  18 ′ upward through the first and second openings  301  and  341 , respectively. In other words, the first and second openings  301  and  341  may be formed directly above the electron emission regions  18 ′ and across from the light emitting unit  16 . 
     The electron emission unit  14 ′ of the electron emission display according to an embodiment of the present invention may also include a second insulating layer  38 . The second insulating layer  38  may be formed on the first insulating layer  30 , such that the plurality of second electrodes  34  may be positioned therebetween. 
     The electron emission unit  14 ′ of the electron emission display according to an embodiment of the present invention may also include a third electrode  36  to function as a focusing electrode. The third electrode  36  may be formed of a single layer and have a predetermined size. The third electrode  36  may be formed on the second insulating layer  38 , such that the second insulating layer  38  may be positioned between the plurality of second electrodes  34  and the third electrode to separate therebetween. 
     The electron emission unit  14 ′ of the electron emission display according to an embodiment of the present invention may further include at least one third opening  381  and at least one fourth opening  361  that may be formed through the second insulating layer  38  and the third electrode  36 , respectively, to provide a path for electron beams from the electron emission regions  18 ′. In particular, the third electrode  36  may have a plurality of fourth openings  361 , such that each fourth opening  361  may be formed to correspond to a respective electron emission region  18 ′ to separately focus electrons emitted therefrom. Alternatively, the third electrode  36  may have a plurality of fourth openings  361 , such that each fourth opening  361  may correspond to a plurality of respective electron emission regions  18 ′ to collectively focus electron beams emitted from more than one electron emission region  18 ′, as illustrated in  FIG. 4 . The at least one third opening  381  and at least one fourth opening  361  may be formed along the length of the third electrode  36 , i.e., y-axis, to expose the plurality of electron emission regions  18 ′ of each pixel unit. 
     The light emission unit  16  as well as the connection between the light emission unit  16  and the electron emission unit  14 ′ may be identical to the description provided with respect to  FIG. 1 . Accordingly, detailed descriptions of elements will not be repeated herein. 
     The at least one spacer  28  may be disposed between the first and second substrates  10  and  12 , as previously discussed with respect to  FIGS. 1-3 . As illustrated in  FIG. 4 , the spacers  28  may be formed as a partition positioned perpendicularly to the first substrate  10  of the electron emission unit  14 ′ and in parallel to the plurality of second electrodes  34 , i.e., the spacers  28  may be formed in the xz-plane. 
     The electron emission display according to an embodiment of the present invention may be driven by application of a predetermined voltage to the plurality of first electrodes  32 , plurality of second electrodes  34 , third electrode  36 , and anode electrode  26 . For example, the first electrodes  32  may function as scan electrodes receiving scan drive voltage, while the second electrodes  34  may function as data electrodes receiving data drive voltage. Alternatively, the functions of the first and second electrodes  32  and  34  may be switched. Further, the third electrode  36  may receive 0V or negative direct current voltage of several to tens volts, while the anode electrode  26  may receive a positive direct current voltage of hundreds to thousands of volts to facilitate acceleration of electron beams. 
     Without intending to be bound by theory, it is believed that application of voltage as described above may provide a voltage difference between the first and second electrodes  32  and  34  that is equal to or higher than a predetermined threshold value, thereby facilitating formation of an electric field around the electron emission regions  18 ′ of each pixel unit having such voltage difference. Formation of such an electric field may, consequently, facilitate emission of electrons from the electron emission regions  18 ′. The emitted electrons may be attracted by the high voltage applied to the anode electrode  26 , pass through the at least one fourth opening  361  of the third electrode  36 , thereby focusing into an electron beam and striking the corresponding phosphor layers  22  to trigger an excitation state thereof and to generate light emission. 
     Some of the electrons emitted from the respective electron emission regions  18 ′ toward the second substrate  12  may collide against the spacers  28 . However, the inventive spacers  28  and  28 ′ according to the embodiment of the present invention may minimize surface charging thereof, thereby reducing distortion of electron beams and preventing abnormal light emission. 
     EXAMPLES 
     An influence of spacers and their specific resistivity characteristics on operation of electron emission displays was tested. More specifically, six (6) spacers were prepared according to embodiments of the present invention. The six (6) inventive spacers and seven (7) conventional spacers were incorporated into thirteen (13) identical electron emission displays. The operation conditions of each electron emission display were identical in terms of voltage, vacuum pressure, and so forth. 
     Each electron emission display was tested to determine whether secondary light emission had occurred during operation thereof. For this purpose, the value of ρ2/ρ1 of the spacers in each electron emission display was modified, and the light emission was observed and recorded. 
     In this respect, secondary light emission refers to undesired light emitted from phosphor layers of the electron emission display due to distortion in the emitted electron beam. More specifically, when electric charge is created around the spacers of an electron emission display and modifies the electric field thereof, the electron beam emitted from the electron emission unit may deviate from its path, thereby causing electrons to impact incorrect surfaces and generate distorted colors and/or images, e.g., showing of a location of a spacer on a screen. 
     The six electron emission displays with the inventive spacers exhibited values of ρ2/ρ1 that correspond to formula 1 of the present invention. Further, the electron emission displays with the inventive spacers did not have secondary light emission occurrence. On the other hand, the seven electron emission displays with the conventional spacers did not correspond to the values of ρ2/ρ1 of the present invention. Further, all the electron emission displays with the conventional spacers exhibited secondary light emission. 
     The lack of secondary light emission occurrence in the electron emission displays with the inventive spacers may be further noted in  FIGS. 6A-6D , where light emitting states of electron emission displays of respective spacers are illustrated. As illustrated in the photographs in  FIGS. 6A-6D , no secondary light emission was observed. On the other hand, as illustrated in  FIGS. 7A-7E  by dotted circles, where light emitting states of electron emission displays of respective conventional spacers are illustrated, secondary light emission was observed. More specifically, it was observed that visibility problems occurred due to appearance of spacers on the screen, i.e., an electron beam path around each spacer was distorted due to the electric charge of the spacer, thereby causing darker areas on the screen. 
     Without intending to be bound by theory, it is believed that secondary light emission as illustrated in  FIGS. 7A-7E  may result when the outermost coating layer has a specific resistivity that is from about 0.01 to about 50 times higher than the resistivity of the element in direct contact with the outer-most coating layer, i.e., an inner coating or a spacer body. In other words, secondary light emission may occur when the spacer is not formed according to formula 1 of the present invention. It is further believed that a resistance at a surface boundary between the outermost coating layer and the element in direct contact with the outer-most coating layer, i.e., an inner coating or a spacer body, may vary quickly, thereby reducing the effectiveness of the coating layer performance in terms of suppressing the spacers surface-charge. 
     Exemplary embodiments of the present invention have been disclosed herein, and although specific terms are employed, they are used and are to be interpreted in a generic and descriptive sense only and not for purpose of limitation. Accordingly, it will be understood by those of ordinary skill in the art that various changes in form and details may be made without departing from the spirit and scope of the present invention as set forth in the following claims.