Abstract:
A structure for a field emission device and a method of making the same. The cathode electrodes are formed as a two-layered structure of chromium on top of aluminum. Photosensitive electron emission material is patterned via back side exposing, the aluminum layer serving as a mask. In receptor regions, conducting aluminum fingers form electrical contact with the emission material while spaces between the aluminum fingers serve as a window to allow for exposure of the photosensitive emission material.

Description:
CLAIM OF PRIORITY  
       [0001]     This application makes reference to, incorporates the same herein, and claims all benefits accruing under 35 U.S.C. § 119 from an application for FIELD EMISSION DISPLAY DEVICE AND MANUFACTURING METHOD OF THE SAME earlier filed in the Korean Intellectual Property Office on 20 Feb. 2004 and there duly assigned Ser. No. 10-2004-0011391.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Technical Field  
         [0003]     The present invention relates to an electron emission device, and in particular, to an electron emission region and a method of manufacturing the same.  
         [0004]     2. Description of the Related Art  
         [0005]     Generally, the 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. The second type of electron emission devices include a field emitter array (FEA) type, a surface conduction emitter (SCE) type, a metal-insulator-metal (MIM) type, a metal-insulator-semiconductor (MIS) type, and a ballistic electron surface emitting (BSE) type.  
         [0006]     The electron emission devices differentiate in their specific structure depending upon the types thereof, but basically have an electron emission unit placed within a vacuum vessel to emit electrons, and an image display unit facing the electron emission unit in the vacuum vessel to emit light or display the desired images.  
         [0007]     With the FEA type electron emission device, electrons are emitted from electron emission regions due to the electric fields formed when driving voltages are applied to the driving electrodes placed around the electron emission regions. The FEA type electron emission device has a triode structure with cathode, gate and anode electrodes. The gate electrodes are first formed on a first substrate, and after an insulating layer is formed on the gate electrodes, cathode electrodes and electron emission regions are formed on the insulating layer.  
         [0008]     With the above structure, there is no possibility that the gate and the cathode electrodes are short-circuited to each other during the processing thereof. As the electron emission regions are placed at the topmost area of the first substrate, a thick filming process, such as screen printing, can be easily applied thereto. The relatively simple processing steps related thereto are advantageous in making wide-screened display devices.  
         [0009]     With the above-structured electron emission device, the electron emission regions are patterned by coating a photosensitive electron emission material onto the first substrate, and exposing it to light, followed by developing it. When ultraviolet rays are illuminate the electron emission material from the front side of the first substrate during the light-exposing step, the pattern of the electron emission regions is non-uniformly made, and the adhesion of the electron emission regions to the cathode electrodes is poor.  
         [0010]     Accordingly, a backside exposure technique where the illumination of ultraviolet rays is made from the back side of the first substrate has been recently applied for the light exposing. However, a structure produced by such a method results in a high contact resistance between the electron emission regions and the cathode electrodes, a large voltage drop across the cathode electrodes and cracks in the insulation layer formed between the cathode and gate electrodes. All of these problems result in deteriorated image quality. What is needed is a design for a field emission device and a method of making that overcomes the above problems.  
       SUMMARY OF THE INVENTION  
       [0011]     It is therefore an object to provide an improved design for a field emission device.  
         [0012]     It is also an object of the present invention to provide a method for making the field emission device.  
         [0013]     It is yet another object of the present invention to provide an electron emission device and a method of manufacturing the same using a backside exposure technique resulting in a device with enhanced device performance characteristics.  
         [0014]     These and other objects may be achieved by a novel structure for a field emission device and a novel method of making the same. The cathode electrodes are formed as a two-layered structure of chromium on top of aluminum. Photosensitive electron emission material is patterned via back side exposing, the aluminum layer serving as a mask. In receptor regions, conducting aluminum fingers form electrical contact with the emission material while spaces between the aluminum fingers serve as a window to allow for exposure of the photosensitive emission material.  
         [0015]     The electron emission device with the following features. The electron emission device includes first and second substrates facing each other, gate electrodes formed on the first substrate, and a metallic layer formed over the gate electrodes with an insulating layer sandwiched in between. The metallic layer has light transmission portions. Cathode electrodes are formed on the metallic layer while being electrically connected to the metallic layer. Electron emission regions are placed within the area of the light transmission portions while being electrically connected to the metallic layer.  
         [0016]     The electron emission regions maybe electrically connected to the cathode electrodes. The light transmission portions may be formed with minute holes partially formed at the metallic layer. The minute holes may be formed with a rectangular, circular, triangular or hexagonal dot type. The minute holes may be vertically or horizontally elongated with a linear shape. The light transmission portions may be formed with a mesh pattern.  
         [0017]     The light transmission portions and the electron emission regions may be formed at one side of the metallic layer and the cathode electrode in the longitudinal direction of the cathode electrode. The cathode electrode may have electron emission region receptors formed by partially removing the cathode electrode. The electron emission region receptor may be formed at the cathode electrode corresponding to the pixel region where the cathode electrode crosses the gate electrode. The metallic layer and the cathode electrode maybe formed with different kinds of metallic materials having etching selectivity.  
         [0018]     The electron emission device may further include counter electrodes arranged on the insulating layer and spaced apart from the electron emission regions with a distance while being electrically connected to the gate electrodes. An electric field reinforcing hole maybe formed at the metallic layer and the cathode electrode opposite to the counter electrode with respect to the electron emission region while being spaced apart from the electron emission region by a distance by removing a part of the metallic layer and the cathode electrode and exposing the gate electrode.  
         [0019]     In a method of manufacturing the electron emission device, gate electrodes are first formed on a first substrate using a transparent conductive material. A transparent dielectric material is then coated onto the first substrate covering the gate electrodes to form an insulating layer thereon. A metallic layer for forming subsidiary cathode electrodes and a metallic layer for forming cathode electrodes are sequentially deposited on the insulating layer. The cathode electrode metallic layer is patterned while exposing the underlying subsidiary cathode electrode metallic layer to form cathode electrodes. The subsidiary cathode electrode metallic layer is patterned to partially form light transmission portions thereon. A photosensitive electron emission material is coated onto the first substrate such that the photosensitive electron emission material is placed within the area of the light transmission portions. The photosensitive electron emission material is exposed by light from the opposite side of the first substrate, and the light-exposed electron emission material is developed to form electron emission regions electrically connected to the subsidiary cathode electrode metallic layer. The subsidiary cathode electrode metallic layer is partially removed while exposing the insulating layer. The degree of light exposure for the photosensitive electron emission material may be controlled such that the photosensitive electron emission material is placed on the subsidiary cathode electrode metallic layer, and electrically connected to the cathode electrodes.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]     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:  
         [0021]      FIG. 1  is a partial exploded perspective view of an electron emission device;  
         [0022]      FIG. 2  is a partial exploded perspective view of an electron emission device according to an embodiment of the present invention;  
         [0023]      FIGS. 3A  to  3 E are plan views of variants of light transmission portions and conductive portions for the electron emission device according to the embodiment of the present invention;  
         [0024]      FIGS. 4A  to  4 E schematically illustrate a method of manufacturing the electron emission device according to the embodiment of the present invention; and  
         [0025]      FIG. 5  is a partial exploded perspective view of an electron emission device according to another embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0026]     An electron emission device where the electron emission regions are made using the backside exposure technique is illustrated in  FIG. 1 . As illustrated in  FIG. 1 , transparent gate electrodes  104  are formed on a first transparent substrate  102 . An insulating layer  106  is formed on the entire inner surface of the first substrate  102  using a transparent material such that it covers the gate electrodes  104 . Cathode electrodes  108  are formed on the insulating layer  106  by coating a metallic material, such as chromium Cr, thereon, and patterning it.  
         [0027]     Electron emission regions  110  are formed at the lateral side of the respective cathode electrodes  108  by coating a carbonaceous and/or nano-sized electron emission material on the insulating layer  106 , and exposing it to light using the backside exposure technique. Specifically, the formation of the electron emission regions  110  is made by first forming a sacrificial layer (not illustrated) on the cathode electrodes  108  and on the insulating layer  106 . The sacrificial layer is patterned to open the locations to be formed with the electron emission regions  110 . A photosensitive electron emission material is coated onto the sacrificial layer, and ultraviolet rays are illuminated thereon through the backside of the first substrate  102 , followed by developing the exposed photosensitive electron emission material by removing the non-hardened electron emission material.  
         [0028]     An anode electrode  114  is formed on the inner surface of the second substrate  112  facing the first substrate  102 , and a phosphor screen  116  is formed on the anode electrode  114  with red, green and blue phosphor layers  116   a  and black layers  116   b.    
         [0029]     With the above structured electron emission device, as the backside exposure technique is used to process the electron emission material, a separate mask is not required, and as the cross-linking of the photosensitive material due to the ultraviolet light is made from the bottom of the electron emission regions, the risk of detachment of the electron emission material during the developing is decreased.  
         [0030]     However, the electron emission regions left over after the developing contact the cathode electrodes only at the lateral side thereof, and after the surface-treatment for enhancing the electron emission, the contact area between the electron emission regions and the cathode electrodes is further reduced. Accordingly, the resulting electron emission device involves increased contact resistance between the electron emission regions and the cathode electrodes, uneven electron emission, and increased driving voltage. In a serious case, the electron emission regions and the cathode electrodes may be electrically short-circuited with each other.  
         [0031]     Furthermore, when the chromium-based layer for forming the cathode electrodes are patterned using an etchant, the insulating layer is liable to be damaged due to the chromium etchant while generating cracks at the surface of the insulating layer. As a result, electron emission material is left over at the cracks of the insulating layer, and unnecessary diode light emission is made during the driving of the display device, thus deteriorating the screen image quality.  
         [0032]     In addition, chromium has a lower conductivity compared to other electrode materials, such as aluminum, and a voltage drop is likely to occur along and across the cathode electrodes, especially in a wide-screened display device. In this case, the amount of electrons emitted from the electron emission regions is decreased due to the voltage drop along the cathode electrodes so that the screen brightness is lowered, and the driving voltage is heightened.  
         [0033]     Turning now to  FIG. 2 ,  FIG. 2  is a partial exploded perspective view of an electron emission device according to an embodiment of the present invention. As illustrated in  FIG. 2 , the electron emission device includes first and second substrates  12  and  14  facing each other. The first and the second substrates  12  and  14  are attached to each other at their periphery with a sealant (ex. frit), thus forming a vacuum vessel. An electron emission structure is provided at the first substrate  12  to emit electrons, and a light emission or display structure at the second substrate  14  to emit visible rays and display the desired images.  
         [0034]     Specifically, gate electrodes  16  are formed on the first substrate  12  with a stripe pattern while proceeding in a direction (in the Y direction of the drawing). An insulating layer  18  is formed on the entire inner surface of the first substrate  12  covering the gate electrodes  16 . Cathode electrodes  20  are formed on the insulating layer  18  and proceed in the direction crossing the gate electrodes  16  (in the X direction of the drawing). Receptors  20 ′ are formed at the cathode electrodes  20 , and electron emission regions  22  are placed within the receptors  20 ′ of the cathode electrodes  20 . The receptors  20 ′ are formed by removing a part of the cathode electrode  20 .  
         [0035]     The gate electrodes  16  are formed with a transparent conductive material, such as indium tin oxide (ITO), and the insulating layer  18  with a transparent dielectric material. The electron emission regions  22  may be stripe-patterned along the cathode electrodes  20 , or arranged at the pixel regions where the gate and the cathode electrodes  16  and  20  cross each other. The electron emission regions  22  are formed with a carbonaceous material, such as carbon nanotube, graphite, diamond, diamond-like carbon, C60 (fulleren), or a combination thereof. The electron emission regions  22  may be formed with a nanometer-sized material, such as carbon nanotube, graphite nanofiber, silicon nanowire, and a combination thereof.  
         [0036]     A subsidiary cathode electrode metallic layer  24  is further provided between the cathode electrodes  20  and the insulating layer  18 . The metallic layer  24  makes it possible to form the electron emission regions  22  using a backside exposure technique, and serves to apply cathode voltages to the electron emission regions  22 .  
         [0037]     With the formation of the electron emission regions  22 , the metallic layer  24  has light transmission portions  24   a  for passing the light for light-exposing the electron emission regions  22 . The light transmission portions  24   a  are placed between the linear (vertically long) conductive portions  24   b  arranged in the direction of the gate electrodes  16  (in the Y direction of the drawing) while being spaced apart from each other.  
         [0038]     The electron emission region  22  is located at the area of the light transmission portions  24   a  within the receptor  20 ′, and contacts the conductive portions  24   b  to receive the cathode voltages therethrough. Moreover, the electron emission regions  22  maybe arranged within the receptors  20 ′ over the conductive portions  24   b  while contacting the cathode electrodes  20 .  FIG. 2  illustrates the case where the electron emission regions  22  are arranged within the receptors  20 ′ while contacting the conductive portions  24   b  of the metallic layer  24  and the cathode electrodes  20 . The arrangement structure of the electron emission regions  22  is not limited thereto, but may be varied provided that they contact the conductive portions  24   b  of the metallic layer  24 .  
         [0039]     Turning now to  FIGS. 3A through 3E ,  FIGS. 3A  to  3 E illustrate variants of the light transmission portions and the conductive portions.  FIGS. 3A  to  3 D illustrate rectangular, circular, triangular and hexagonal dot-typed light transmission portions  20   c,    20   e,    20   g  and  20   i  respectively, and conductive portions  20   d,    20   f,    20   h  and  20   j  for forming the light transmission portions  20   c,    20   e,    20   g  and  20   i,  respectively. That is, with those variants, the light transmission portions  20   c,    20   e,    20   g  and  20   i  are formed with a mesh type.  
         [0040]      FIG. 3E  illustrates light transmission portions  20   k  arranged in the direction of the cathode electrodes  20  (in the X direction of the drawing) and linear (horizontally long) conductive portions  201  for forming the light transmission portions  20   k.  As before, the light transmission portions and the conductive portions may be altered with various shapes.  
         [0041]     Meanwhile, the metallic layer  24  and the cathode electrodes  20  are preferably formed with different kinds of metals with etching selectivity. Particularly in this embodiment, the metallic layer  24  contacting the insulating layer  18  is formed with aluminum Al having excellent conductivity, and the cathode electrodes  20  facing the second substrate  14  with chromium Cr having excellent durability.  
         [0042]     An anode electrode  26  is formed on the surface of the second substrate  14  facing the first substrate  12 , and a phosphor screen  28  is formed on the anode electrode  26  with red, green and blue phosphor layers  28   a  and black layers  28   b.  A metal-based reflective layer, for instance, an aluminum-based reflective layer, may be formed on the phosphor screen  28 .  
         [0043]     The above-structured electron emission device is driven by applying predetermined voltages to the gate electrodes  16 , the cathode electrodes  20  and the anode electrode  26  from the outside. For instance, a positive (+) voltage of several to several tens volts is applied to the gate electrodes  16 , a negative (−) voltage of several to several tens volts to the cathode electrodes  20 , and a positive (+) voltage of several hundreds to several thousands volts to the anode electrode  26 .  
         [0044]     Electric fields are formed around the electron emission regions  22  due the voltage difference between the gate and the cathode electrodes  16  and  20 , and electrons are emitted from the electron emission regions  22  due to the electric fields. The emitted electrons are attracted toward the phosphor screen  28  by the high voltage applied to the anode electrode  26 , and land on the phosphor layers  28   a  at the relevant pixels, thus causing them to emit light to display the desired images.  
         [0045]     A method of manufacturing the electron emission device will be now explained. Turning now to  FIGS. 4A through 4C ,  FIGS. 4A  to  4 C illustrate the method of manufacturing the electron emission device.  
         [0046]     A transparent conductive material, such as indium tin oxide (ITO), is coated onto the first transparent substrate  12 , and is patterned to form stripe-shaped gate electrodes  16 . A transparent dielectric material is printed onto the top surface of the first substrate  12  and over the gate electrodes  16 , and is dried and is fired to thus form an insulating layer  18 . Thereafter, an aluminum Al layer is deposited onto the insulating layer  18  to a thickness of 50-1,000 nm to form a first precursor layer  24 ′ for making the metallic layer  24 . A chromium Cr layer is deposited onto the first precursor layer  24 ′ to a thickness of 50-1,000 nm to form a second precursor layer  20 ″ for making the cathode electrodes. As illustrated in  FIG. 4A , the first and the second precursor layers  24 ′ and  20 ″ may be formed using a thin film formation process, such as sputtering and dipping.  
         [0047]     The second precursor layer  20 ″ is patterned using a mask (not illustrated) and a chromium etchant to form stripe-shaped cathode electrodes  20  proceeding in the direction crossing the gate electrodes  16 . At this time, receptors  20 ′ are also simultaneously patterned and formed. As illustrated in  FIG. 4B , with the patterning of the second precursor layer  20 ″, the first precursor layer  24 ′ still covers the entire surface of the insulating layer  18 , thus preventing the insulating layer  18  from being damaged due to the chromium etchant used to etch the second precursor layer  20 ″.  
         [0048]     As illustrated in  FIG. 4C , the first precursor layer  24 ′ is patterned using a mask (not illustrated), thus forming light transmission portions  24   a  and conductive portions  24   b  within the receptors  20 ′. A paste-phased photosensitive electron emission material  22 ′, mainly containing carbon nanotube, is printed onto the cathode electrodes  20  to a large thickness. At this time, the photosensitive electron emission material  22 ′ is placed within the receptors  20 ′, that is, at the light transmission portions  24   a  and the conductive portions  24   b.    
         [0049]     After the printing of the photosensitive electron emission material  22 ′, portions of the photosensitive electron emission material  22 ′ is exposed from the back side using ultraviolet light as illustrated by the arrows in  FIG. 4D . During this exposure, metal layer  24  serves as a mask during the exposure. The photosensitive electron emission material  22 ′ placed at the light transmission portions  24   a  is first hardened, and as the ultraviolet illumination degree is reinforced, the electron emission material  22 ′ placed on the conductive portions  24   b  while contacting the cathode electrodes  20  is also hardened. Thereafter, as illustrated in  FIG. 4E , the non-hardened electron emission material  22 ′ (i.e., the portions of the electron emission material not exposed by the ultraviolet light because of metal layer  24 ) as well as the unnecessary portions of the second precursor layer  24 ′ are removed, and electron emission regions  22  with the desired pattern are formed.  
         [0050]     Spacers (not illustrated) are mounted onto the first substrate  12 , and the first substrate  12  is attached to the second substrate  14  having the anode electrode  26  and the phosphor screen  28  using a sealant (not illustrated). The inner space between the first and the second substrates  12  and  14  is evacuated to thus complete the formation of the electron emission device.  
         [0051]     Turning now to  FIG. 5 ,  FIG. 5  illustrates a field emission device according to a second embodiment of the present invention. As illustrated in  FIG. 5 , the electron emission device may further include counter electrodes  30  for pulling up the electric field formed by the gate electrodes  16  over the insulating layer  18 . The counter electrodes  30  contact the gate electrodes  16  through holes (not illustrated) formed in the insulating layer  18  while being electrically connected thereto. The counter electrodes  30  are spaced apart from the electron emission regions  22  and between neighboring cathode electrode by a distance.  
         [0052]     When predetermined driving voltages are applied to the gate electrodes  16  and the cathode electrodes  20  to form electric fields between them, the counter electrodes  30  make the electric fields diffuse around the electron emission regions  22  such that stronger electric fields are applied to the electron emission regions  22 , causing electrons to be better emitted from the electron emission regions  22 . As with the cathode electrodes  20 , the counter electrodes  30  are laminated with an aluminum-based first layer  30   a  corresponding to the metallic layer  24  and a chromium-based second layer  30   b  corresponding to the cathode electrodes  20 .  
         [0053]     Furthermore, electric field reinforcing holes  32  are formed within the cathode electrodes  20  in the direction facing the counter electrodes  30  around the electron emission regions  22  by partially removing the cathode electrodes  20  and the metallic layer  24 . The holes  32  are operated similar to the counter electrodes  30 .  
         [0054]     As described above, with the inventive electron emission device, the subsidiary cathode electrode metallic layer  24  prevents the insulating layer  18  from being damaged due to the chromium etchant, thus preventing the generation of cracks on the surface of the insulating layer  18 . Accordingly, the diode light emission made due to the remnants of the electron emission material  22  at the cracks of the insulating layer  18  is prevented, thus enhancing the screen image quality.  
         [0055]     Furthermore, as the subsidiary cathode electrode metallic layer  24  with an excellent conductivity heightens the conductivity of the cathode electrodes  20 , the voltage drop across the cathode electrodes  20  is reduced while increasing the amount of electrons emitted from the electron emission regions  22 , thus enhancing the screen brightness while allowing for low driving voltages driving.  
         [0056]     In addition, as the electron emission regions  22  are electrically connected to the metallic layer  24  placed under the cathode electrodes  20 , the shortcomings made due to the electrical contact failure between the cathode electrodes  20  and the electron emission regions  22  can be removed. As the resistance values of the respective pixels are the same, the distortion in the driving signals due to the electrode resistance is reduced, and the non-uniformity in the amount of charged electrical currents due to the unevenly decreased driving voltage can be removed.  
         [0057]     Meanwhile, it is explained that gate electrodes  16  are formed with a stripe pattern, and an anode electrode  30  is formed on the entire inner surface of the second substrate  14 . Alternatively, it is possible that a gate electrode  16  is formed on the entire inner surface of the first substrate  12 , and anode electrodes  30  are formed with a striped pattern while proceeding in the direction crossing the cathode electrodes  20 .  
         [0058]     Although preferred embodiments of the present invention have been described in detail hereinabove, it should be clearly understood that many variations and/or modifications of the basic inventive concept herein taught which may appear to those skilled in the art will still fall within the spirit and scope of the present invention, as defined in the appended claims.