Abstract:
An electron emission device adapted to enhanced electron beam focusing and its method of fabrication are shown. The device includes driving electrodes for controlling the emission of electrons from electron emission regions formed on a substrate; two or more tiers of insulating layers formed on the driving electrodes; and a focusing electrode formed over the tiers. A multi-tiered insulating layer allows a thick tier to hold the focusing electrodes away from the emission regions, thus enhancing their focusing impact, while a thin tier under the focusing electrodes remains amenable to intricate patterning. Fabrication of the tiers from material with different etching rates allows thicker lower support tiers to be etched during the same period and in the same step that a thinner upper tier is etched, also allowing openings in a lower tier to widen while openings in the upper tier stay small.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   This application claims priority to and the benefit of Korean Patent Application No. 10-2004-0038238 filed on May 28, 2004in the Korean Intellectual Property Office, the entire content of which is incorporated herein by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an electron emission device, and in particular, to an electron emission device and a method of manufacturing the same which enhances the structure of a focusing electrode for controlling the electron beams and an insulating layer for supporting the focusing electrode. 
   2. Description of Related Art 
   Generally, electron emission devices are classified into a first type where a hot cathode is used as an electron emission source, and a second type where a cold cathode is used as the electron emission source. 
   Cold cathode electron emission devices include, for example, field emitter array (FEA) devices, surface conduction emitter (SCE) devices, metal-insulator-metal (MIM) devices, metal-insulator-semiconductor (MIS) devices, and ballistic electron surface emitting (BSE) devices. 
   Electron emission devices vary in their structure depending upon the specific type of the device. However, most have a basic structure including a vacuum chamber formed by two substrates, electron emission regions and driving electrodes that are formed on one of the substrates, and phosphor layers that are formed on the other substrate. The driving electrodes help emit electrons from the electron emission regions and phosphor layers emit light to display the desired images. 
   In an electron emission device with the above general structure, correcting the trajectory of electron beams to enhance the display characteristics has been a challenge. For example, electrons emitted from the electron emission regions on one of the substrates may diffuse before colliding against the phosphor layers on the other substrate. As a result, the diffused electrons do not strike the intended phosphor layers; instead, they land on other-neighboring phosphor layers causing them to emit an unintended color. 
   Metallic mesh-shaped grid electrodes or focusing electrodes have been used to control the trajectory of the electron beams. A grid electrode is placed between the two substrates while set apart from them using spacers. Focusing electrodes are located over the first substrate, which includes the electron emission regions, and surround the electron emission regions. 
   Fabrication of electron emission devices using grid electrodes involves difficult and complicated processing steps. At first, spacers are mounted on one of the two substrates; then, the grid electrode is aligned to the substrates; and then, the substrates are attached to each other to form a vacuum chamber. 
   Effective use of focusing electrodes may also lead to difficulty in the required fabrication process. The electron beam focusing effect of a focusing electrode is enhanced if the focusing electrode is set at a distance from the electron emission regions. To set the focusing electrode away from the electron emission regions, the thickness of the insulating layer, that supports the focusing electrode, must increase. An increased insulator thickness, in turn, results in longer and deeper opening portions, passage wells or holes through the insulator layer to the electron emission regions on the substrate. Forming holes with a high vertical to horizontal ratio involves fabrication processing difficulties. For example, if a wet etch process is used to form a hole, the etchant may tend to widen the hole as it deepens it. Therefore, achieving a deep hole while keeping the width small is not trivial. 
   SUMMARY OF THE INVENTION 
   In one exemplary embodiment of the present invention, there are provided an electron emission device and a method of manufacturing the same which improve the structure of a focusing electrode and an insulating layer for supporting the focusing electrode to thereby enhance the electron beam focusing effect. 
   In an exemplary embodiment of the present invention, an electron emission device includes one or more driving electrodes for controlling the emission of electrons from electron emission regions formed on a substrate. Two or more insulating layers are formed on the driving electrodes, and a focusing electrode is formed on the insulating layers. The insulating layers have opening portions exposing the electron emission regions on the substrate, and the opening portions of the insulating layers are differentiated in size from each other. 
   The insulating layer contacting the driving electrodes has a first opening portion, and the insulating layer contacting the focusing electrode has a plurality of second opening portions smaller than the first opening portion. The plurality of second opening portions are arranged within the area of the first opening portion. The insulating layers are differentiated in etching rate from each other, and the etching rate of the insulating layer placed apart from the focusing electrode is greater than the etching rate of the insulating layer placed close to the focusing electrode. 
   In another exemplary embodiment of the present invention, an electron emission device includes first and second substrates facing each other, and cathode and gate electrodes placed on the first substrate while being insulated from each other by interposing a lower insulating layer. Electron emission regions are electrically coupled to the cathode electrodes. A focusing electrode is placed on the electron emission regions while surrounding the electron emission regions. Two or more insulating layers are placed under the focusing electrode while supporting the focusing electrode. The insulating layers are based on different kinds of insulating materials with opening portions exposing the electron emission regions on the first substrate, and the opening portions of the insulating layers are differentiated in size from each other. 
   In a method of manufacturing the electron emission device, cathode and gate electrodes are first formed on a substrate. An insulating layer with a relatively high etching rate and an insulating layer with a relatively low etching rate are sequentially deposited onto the electrodes to form two or more insulating layers differentiated in etching rate from each other. A focusing electrode is formed on the insulating layers such that the focusing electrode has an opening portion with a predetermined size. The insulating layers are etched using the focusing electrode as a mask layer to thereby form an opening portion with a relatively large width at the insulating layer placed apart from the focusing electrode while forming a plurality of opening portions with relatively small widths at the insulating layer contacting the focusing electrode. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a partial perspective view of an electron emission device according to one embodiment of the present invention. 
       FIG. 2  is a partial cross-sectional view of the electron emission device shown in  FIG. 1 . 
       FIG. 3  is a partial cross-sectional view of an electron emission device according to another embodiment of the present invention. 
       FIG. 4  is a partial plan view of the electron emission device shown in  FIG. 3 . 
       FIGS. 5A to 5D  illustrate exemplary fabrication steps of an electron emission device of the present invention. 
   

   DETAILED DESCRIPTION 
   As shown in  FIG. 1  and  FIG. 2 , one embodiment of the electron emission device of this invention  100  includes a first substrate  2  and a second substrate  4 . The substrates  2 ,  4  are arranged in parallel while being apart from each other, leaving a space in between. The substrates  2 ,  4  are attached to each other by a spacer, to form a vacuum chamber outlining the device. 
   Cathode electrodes  6  may be formed with a stripe pattern on the first substrate  2  along one of the axes of the substrate. In  FIG. 1 , for example, the cathode electrodes  6  are formed in stripes along the y-axis of the drawing. A lower insulating layer  8  may be formed over the first substrate  2  covering the cathode electrodes  6 . A number of gate electrodes  10  may be formed on the lower insulating layer  8 . The gate electrodes  10  may be formed with a stripe pattern proceeding along a direction perpendicular to the direction of cathode electrodes  6 . In  FIG. 1 , for example, the gate electrodes  10  are formed in stripes along the x-axis of the drawing. 
   In the embodiment shown in  FIG. 1  and  FIG. 2 , regions where the cathodes  6  and the gate electrodes  10  cross paths may be defined as pixel regions. A number of electron emission regions  12  are formed on the cathode electrodes  6  at these pixel regions. Gate wells or holes  8   a ,  10   a  are formed through the first insulating layer  8  and the gate electrodes  10 . The gate holes  8   a ,  10   a  correspond to the electron emission regions  12  and expose the electron emission regions  12  to the vacuum chamber formed between the two substrates  2 ,  4 . 
   In one embodiment, shown in  FIG. 1  and  FIG. 2 , the electron emission regions  12  are linearly arranged along the longitudinal direction of the cathode electrodes  6  in the pixel regions. If the electron emission regions  12  are formed to have a rectangular shape, then, the gate holes  8   a ,  10   a  may also be rectangular to correspond to electron emission regions  12  in plan view. 
   The electron emission regions  12  may be formed from a carbonaceous material or a nanometer-sized material that emit electrons when an electric field is applied to them. For example, the electron emission regions  12  may be formed with carbon nanotube, graphite, graphite nanofiber, diamond, diamond-like carbon, C 60 , silicon nanowire, a combination of the foregoing, or any like material. The electron emission regions  12  may be formed through direct growth, screen printing, chemical vapor deposition, sputtering, or similar processes. 
   In the embodiment shown on  FIG. 1  and  FIG. 2 , the cathode electrodes  6  and the gate electrodes  10  are insulated from each other by the lower insulating layer  8 . In this embodiment, the gate electrodes  10  surround the electron emission regions  12 . When driving voltages are applied to the cathode electrodes  6  and gate electrodes  10 , electric fields are formed around the electron emission regions  12 . The electric fields created by the voltage difference between the cathode and gate electrodes  6 ,  10 , cause the electron emission regions  12  to emit electrons. 
   In the embodiment shown in  FIG. 1  and  FIG. 2 , the gate electrodes  10  are placed above the cathode electrodes  6  with the lower insulating layer  8  separating the cathode electrodes  6  from the gate electrodes  10 . Alternatively, in another embodiment (not shown), the gate electrodes  10  may be placed under the cathode electrodes  6  while the two are separated by the lower insulating layer  8 . In this case, the electron emission regions  12  may be formed on one-side of a periphery of the cathode electrodes  6 . 
   As shown in  FIG. 1  and  FIG. 2 , an upper insulating layer  14  and a focusing electrode  16  are formed or placed over the gate electrodes  10  and the lower insulating layer  8 . In the embodiment shown, opening portions  18   a ,  20   a  and  16   a  are formed for exposing the electron emission regions  12  to the inside of the vacuum chamber formed between the substrates  2 ,  4 . The focusing electrode  16  are formed over the entire extent of the first substrate  2 . Alternatively, the focusing electrode  16  may be divided into a number of portions with a predetermined pattern. The focusing electrode  16  may be formed from a metallic thin film by depositing a metallic material on the upper insulating layer  14 . In another embodiment, the focusing electrode  16  may be formed by attaching a metal plate with opening portions  16   a  to the upper insulating layer  14 . 
   The focusing electrode  16  is capable of focusing the electrons emitted from the electron emission regions  12 , and when a high voltage is applied to the second substrate  4 , prevents the electron emission regions  12  from being influenced by the electric field due to the high voltage. The gate electrode  10  and the focusing electrode  16  are separated by the upper insulating layer  14  to prevent the gate and focusing electrodes  10 ,  16  from contacting with each other and creating a short-circuit. The beam focusing effect of the focusing electrode  16  is enhanced as the thickness of the upper insulating layer  14  is increased. 
   In one embodiment, the upper insulating layer  14  may have a two-tiered structure with a first or lower tier  18  and a second or upper tier  20 . Opening portions  18   a ,  20   a , that may be holes, are formed through tiers  18 ,  20  of the double-tiered upper insulating layer  14  for exposing the electron emission regions  12  to the vacuum chamber. Each part of an opening portion  18   a ,  20   a  may have a different thickness or depth corresponding to the different thicknesses of the tiers  18 ,  20  of the double-tiered upper insulating layer  14 . A relatively long opening portion  18   a  may be formed through the first or lower tier  18  of the insulating layer  14 . A relatively short opening portion  20   a  may be formed through the second or upper tier  20  of the upper insulating layer  14  directed toward the focusing electrode  16 . As a result, in this embodiment, the focusing electrode  16  will have a sufficient distance from the electron emission regions  12 . 
   More than two tiers may be used to create the upper insulating layer  14 . A several-tiered upper insulating layer  14  may be formed by depositing a sequence of insulating layers of different thickness and characteristics. 
   In one embodiment, the upper insulating layer  14  may have a laminated structure of the first or lower insulating tier  18  and the second or upper insulating tier  20 . When an opening portion  18   a  is formed through the first insulating tier  18 , one or more opening portions  20   a ,  16   a  may be formed through the second insulating tier  20  and the focusing electrode  16  corresponding to the opening portion  18   a . The opening portion  18   a  of the first insulating tier  18  may be formed at pixel regions. Then, one or more opening portions  20   a ,  16   a  are formed through the second insulating tier  20  and the focusing electrode  16 , also, at each pixel region. So, one opening portion  18   a  of the first or lower insulating tier  18  may correspond to several opening portions  20   a ,  16   a  through the second or upper insulating tier  20  at each pixel region. 
   In some embodiments, the opening portions  18   a  formed in the first insulating tier  18  of the upper insulating layer  14  may have a larger cross-sectional area than the opening portions  20   a  formed in the second insulating tier  20 . In these embodiments, the first or lower tier  18  functions as support for the second or upper tier  20  and for the focusing electrode  16 . The second insulating tier  20  has opening portions  20   a  with smaller cross-sectional areas. The smaller area of the second or upper opening portions  20   a , allows intricate patterns for the opening portions  16   a , of the focusing electrode  16 , that are formed over opening portions  20   a . Again, considering the respective functions of the first and the second insulating tiers  18 ,  20 , the first or lower insulating tier  18  is usually formed with a larger thickness and the second or upper insulating tier  20  is formed with a smaller thickness. In some embodiments, the thickness of the first insulating tier  18  may be one to five times greater than the thickness of the second insulating tier  20 . 
   In some embodiments, the first and the second insulating tiers  18 ,  20  are formed with different kinds of materials, which exhibit different etching rates with respect to an etching solution or an etching gas. In these embodiments, the upper insulating layer  14  can be easily removed to create the required opening portions  18   a ,  20   a  through one etching process. For example, if the thickness of first insulating tier  18  is greater than the thickness of the second insulating tier  20 , then if the etching rate of the first insulating tier  18  is also greater than that of the second insulating tier  20 , the two layers may be removed in one etch step. For example, when it is intended to etch through the upper insulating layer  14  by one wet etching process and the first insulating tier  18  is ten to twenty times as thick as the second insulating tier  20 , then the etching rate of the first insulating tier  18  may be established to be ten to twenty times greater than that of the second insulating tier  20 . 
   In embodiments where more than two tiers are used to form the upper insulating layer  14 , the respective functions of each tier determine the thickness and the etch rate of each tier. 
   As seen in  FIG. 1  and  FIG. 2 , red, green, and blue phosphor layers  22  are formed on the surface of the second substrate  4  facing the first substrate  2 . A black layer  24  is formed between the neighboring phosphor layers  22 . An anode electrode  26  is formed on the phosphor layers  22  and the black layers  24 . The anode electrode  26  may be formed with a metallic layer, for example, an aluminum layer formed through deposition. The anode electrode  26  is coupled to a high voltage from outside to accelerate electron beams. The anode electrode  26  may also reflect some of the visible rays radiated toward the first substrate  2  back toward the second substrate  4 , thereby heightening the screen brightness. 
   In one embodiment (not shown), the anode electrode  26  may be formed with a transparent conductive material, such as indium tin oxide (ITO). In this embodiment, the anode electrode  26  is formed under the phosphor layers  22  and the black layers  24  and directly on the second substrate  4 . This anode electrode  26  may be formed on the entire surface of the second substrate  4 , or divided into a number of portions with a predetermined pattern covering only parts of the second substrate  4 . 
   As described above, the upper insulating layer  14  for supporting the focusing electrode  16  is formed over a laminated structure of first and second insulating tiers  18 ,  20 . The fist and second insulating tiers  18 ,  20  have opening portions  18   a ,  20   a  with different thicknesses and different cross-sectional areas so that the focusing electrode  16  has a sufficient height with respect to the electron emission regions  12 , and the opening portions  16   a  of the focusing electrode  16  may be minutely patterned. 
   Consequently, the electron emission device  100  involves an enhanced electron beam focusing effect, and shields the anode electric field with respect to the electron emission regions  12  more effectively, thereby preventing the unintended light emission. 
     FIG. 1  and  FIG. 2 , show an embodiment of the electron emission device  100  where for every electron emission region  12  and its corresponding gate hole  8   a ,  10   a  , there is one opening portion  16   a ,  20   a  formed through the focusing electrode  16  and the second insulating tier  20 . 
     FIG. 3  and  FIG. 4  show an alternative embodiment of the electron emission device  200  where for every electron emission region  12  and its corresponding gate hole  8   a ,  10   a , there are a number of opening portions  16   a ′,  20   a ′ formed through the focusing electrode  16 ′ and the second insulating tier  20 ′. 
   A method of manufacturing the electron emission device  100 ,  200  will be now explained with reference to  FIGS. 5A to 5D . 
   As shown in  FIG. 5   a , cathode electrodes  6 , a lower insulating layer  8  and gate electrodes  10  are sequentially formed on a first substrate  2 , and at least one gate hole  8   a ,  10   a  is formed through the lower insulating layer  8  and the gate electrode  10  per each pixel region such that the cathode electrode  6  is partially exposed. 
   First and second insulating tiers  18 ,  20 , forming an upper insulating layer  14 , are deposited onto the surface of the first substrate  2 , over the gate electrodes  10  and the lower insulating layer  8 . The first and the second insulating tiers  18 ,  20  are formed with materials with different etch rates with respect to an etching solution or an etching gas. For example, the etching rate of the first insulating tier  18  may be ten to twenty times greater than that of the second insulating tier  20 . 
   The first insulating tier  18  supports the focusing electrode  16  to be formed later and may repeatedly suffer printing and firing. For example, the thickness of the first insulating tier  18  may vary from several micrometers to tens of micrometers. The second insulating tier  20  has a role of forming minute opening portions  20   a  adjacent to the focusing electrode  16  to be formed later. For example, the thickness of the second insulating tier  20  may be several micrometers to tens of micrometers. 
   As shown in  FIG. 5B , a focusing electrode  16  is formed on the second insulating tier  20  with opening portions  16   a . For formation of the focusing electrode  16 , a metallic material may be deposited onto the second insulating tier  20 , and patterned. For this purpose, a thin metal plate including opening portions  16   a , may be attached to the second insulating tier  20 . 
   As shown in  FIG. 5B , the opening portions  16   a  of the focusing electrode  16  are arranged in one to one correspondence with the gate holes  8   a ,  10   a  of the lower insulating layer  8  and the gate electrodes  10 . As shown in  FIGS. 3 and 4 , a number of opening portions  16   a  may correspond to one gate hole  8   a ,  10   a.    
   Thereafter, as shown in  FIG. 5C , the upper insulating layer  14  are etched using the focusing electrode  16  as a mask layer. A wet etching process may be used for the etching. When the upper insulating layer  14  is etched using the focusing electrode  16  as a mask layer, a number of opening portions  20   a  are formed through the second insulating tier  20  in conformity with the shape of the focusing electrode  16 . At the same time, the first insulating tier  18  is over-etched so that the opening portions  18   a  are interconnected forming one large opening volume. 
   Consequently, in an embodiment shown in  FIG. 5D , the first insulating tier  18  has an opening portion  18   a  with a large width or cross-sectional area, and the second insulating tier  20  and the focusing electrode  16  have a number of opening portions  20   a ,  16   a  over the opening portion  18   a  of the first insulating tier  18 . 
   Finally, a paste containing an electron emission material and a photosensitive material is screen-printed onto the cathode electrodes  6 , and exposed to light, followed by developing and firing to form electron emission regions  12  on the cathode electrodes  6 . 
   The first substrate  2 , with an electron emission structure, faces a second substrate  4 , with phosphor layers  22  and an anode electrode  26 , and the two substrates  2 ,  4  are separated by a predetermined distance. The two substrates  2 ,  4  are attached by using a sealing material, such as a frit. The inner space between the first and the second substrates  2 ,  4  is partially exhausted and kept in a partial vacuum state, thereby forming an electron emission device. 
   As described above, with the electron emission device of this invention  100 ,  200 , the focusing electrode  16  has a sufficient height with respect to the electron emission regions  12 , and the opening portions  16   a ,  16   a ′ of the focusing electrode  16  are small. Accordingly, the electron beam focusing effect by way of the focusing electrode  16  is enhanced, and the anode electric field with respect to the electron emission regions  12  is intercepted more effectively. 
   Although exemplary embodiments of the present invention have been shown and described, those skilled in the art would appreciate that changes may be made in the embodiments without departing from the principles and spirit of the invention, the scope of which is defined in the claims and their equivalents.