Patent Publication Number: US-7581999-B2

Title: Electron emission device having openings with improved aspect ratio and method of manufacturing

Description:
CROSS REFERENCE TO RELATED APPLICATION 
   This application claims priority to and the benefit of Korean Patent Application No. 10-2004-0060600 filed on Jul. 30, 2004 in 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 embodiments of the present invention relate to an electron emission device and a method of manufacturing the device, and in particular, to an electron emission device having an insulating layer with high aspect ratio openings and a manufacturing method thereof. 
   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. 
   Among the second type of electron emission devices are devices known as field emitter array (FEA) type devices, surface-conduction emission (SCE) type devices, metal-insulator-metal (MIM) type devices, and metal-insulator-semiconductor (MIS) type devices. 
   The FEA type electron emission devices are based on the principle that when a material having a low work function or a high aspect ratio is used as the electron emission source, electrons are easily emitted from the material in a vacuum when exposed to an electric field. A sharp-pointed tip structure including molybdenum (Mo) or silicon (Si) or a carbonaceous material, such as carbon nanotube, graphite and diamond-like carbon, has been developed to be used as the electron emission source. 
   In the FEA type electron emission devices, cathode electrodes, an insulating layer and gate electrodes are sequentially formed on a first substrate and openings are formed in the gate electrodes and the insulating layer. Electron emission regions are formed on the cathode electrodes within the openings. Phosphor layers and an anode electrode are formed on a surface of a second substrate facing the first substrate. 
   The insulating layer is formed through repeating a screen printing, drying and firing process several times such that it has a thickness of 5-30 μm. The insulating layer is patterned through wet etching to form openings. With the wet etching, as the etching is made in an isotropic manner, the insulating layer is etched in a horizontal direction as well as in a vertical direction. Accordingly, the width of the opening becomes larger than the depth of the opening, and as a result, the opening has an aspect ratio of 1 or less. 
   Such a phenomenon is more seriously manifested as the thickness of the insulating layer is increased and becomes a critical factor in making it difficult to conduct the micro patterning process. Accordingly, the pixels are not easily arranged on the first substrate in a compact manner and a desired high resolution display screen is not obtainable. As the number of electron emission regions arranged within a predetermined area of pixels is limited, it becomes difficult to emit a suitable amount of electrons. 
   SUMMARY 
   In one exemplary embodiment of the present invention, there is provided an electron emission device and a method of manufacturing the electron emission device, which increases the aspect ratio of the openings formed in the insulating layer to enable the micro patterning, and enables the attainment of high resolution and high luminance. 
   In an exemplary embodiment of the present invention, in a method of manufacturing the electron emission device, cathode electrodes are first formed on a substrate. An insulating layer is formed on the entire surface of the substrate such that the insulating layer covers the cathode electrodes. The insulating layer is wet-etched two or more times to form openings in the insulating layer each with an aspect ratio of more than 1. Gate electrodes are formed on the insulating layer. Electron emission regions are formed on the cathode electrodes within the openings of the insulating layer. 
   The etchings may be conducted using separate mask patterns having the same-sized openings. The etchings may be conducted such that under-cuts are made. 
   In another exemplary embodiment of the present invention, in a method of manufacturing the electron emission device, cathode electrodes are first formed on a substrate. An insulating layer is formed on the entire surface of the substrate such that the insulating layer covers the cathode electrodes. A first mask pattern with openings is formed on the insulating layer. The portions of the insulating layer exposed through the openings of the first mask pattern are first etched by wet etching and the first mask pattern is removed. A second mask pattern is formed on the insulating layer. The second mask pattern has openings with the same size as the openings of the first mask pattern. The portions of the insulating layer exposed through the openings of the second mask pattern are etched a second time by wet etching and the second mask pattern is removed. A third mask pattern is formed on the insulating layer. The third mask pattern has openings with the same size as the openings of the first mask pattern. The portions of the insulating layer exposed through the openings of the third mask pattern are etched a third time by wet etching and the third mask pattern is removed. Gate electrodes are formed on the insulating layer. Electron emission regions are formed on the cathode electrodes within the openings of the insulating layer. 
   The first etching may be conducted to remove ⅓ of the thickness of the insulating layer. The second etching may be conducted to remove ½ of the thickness of the remaining insulating layer. The first, the second and the third etchings may be conducted such that under-cuts are made. 
   In a further exemplary embodiment of the present invention, the electron emission device includes a first and a second substrate facing each other at a predetermined distance, cathode electrodes formed on the first substrate, and electron emission regions formed on the cathode electrodes. An insulating layer is formed on the cathode electrodes with openings having an aspect ratio of more than 1. The openings of the insulating layer are formed at the respective electron emission region formation locations. Gate electrodes are formed on the insulating layer. Phosphor layers are formed on the second substrate. At least one anode electrode is formed on a surface of the phosphor layers. The insulating layer may have a thickness of 5-30 μm. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1A  is a diagram of a first phase in one embodiment of a process of manufacturing an electron emission device. 
       FIG. 1B  is a diagram of a second phase in one embodiment of a process of manufacturing an electron emission device. 
       FIG. 1C  is a diagram of a third phase in one embodiment of a process of manufacturing an electron emission device. 
       FIG. 1D  is a diagram of a fourth phase in one embodiment of a process of manufacturing an electron emission device. 
       FIG. 1E  is a diagram of a fifth phase in one embodiment of a process of manufacturing an electron emission device. 
       FIG. 1F  is a diagram of a sixth phase in one embodiment of a process of manufacturing an electron emission device. 
       FIG. 1G  is a diagram of a seventh phase in one embodiment of a process of manufacturing an electron emission device. 
       FIG. 1H  is a diagram of a eighth phase in one embodiment of a process of manufacturing an electron emission device. 
       FIG. 2  is a partial cross-sectional view of an insulating layer for the electron emission device according to one embodiment of the present invention. 
       FIG. 3  is a partially exploded perspective view of an electron emission device according to one embodiment of the present invention. 
       FIG. 4  is a partial cross-sectional view of the electron emission device according to one embodiment of the present invention. 
   

   DETAILED DESCRIPTION 
   As shown in  FIG. 1A , cathode electrodes  4  are first formed laid out in a first direction on a first substrate  2  and an insulating layer  6  is formed on the entire surface of the first substrate  2  such that it covers the cathode electrodes  4 . The formation of the insulating layer  6  is made by repeating process of screen printing, drying and firing such that the insulating layer  6  has a thickness of 5-30 μm. 
   Thereafter, a first mask pattern with openings  81  is formed on the insulating layer  6 . The first mask pattern  8  may be formed with a photosensitive film, which is patterned through a photolithography process using an exposure mask (not shown) to thereby form openings  81 . 
   As shown in  FIG. 1B , the portions of the insulating layer  6  exposed through the openings  81  of the first mask pattern  8  are wet-etched by a predetermined thickness to thereby form first etched portions  61 . In this process, the etching is performed such that under-cuts are made under the first mask pattern  8 . In this way, the maximum width of the first etched portion  61  is established to be larger than the width of the opening  81  of the first mask pattern  8 . The insulating layer  6  is etched by ⅓ of the previous thickness to thereby form the first etched portions  61 . After the formation of the first etched portions  61 , the first mask pattern  8  is removed. 
   As shown in  FIG. 1C , a second mask pattern  10  with openings  101  is formed on the insulating layer  6 . The second mask pattern  10  may also be formed with a photosensitive film, which is patterned through a photolithography process using an exposure mask (not shown) to thereby form openings  101 . The exposure mask is the same as that used in forming the first mask pattern  8 . Accordingly, the opening  101  of the second mask pattern  10  has the same size as that of the opening  81  of the first mask pattern  8 . The second mask pattern  10  covers the sidewalls of the first etched portions  61 . 
   As shown in  FIG. 1D , the portions of the insulating layer  6  exposed through the openings  101  of the second mask pattern  10  are wet-etched by a predetermined thickness to thereby form second etched portions  62 . Even in this process, the etching is performed such that under-cuts are made under the second mask pattern  10 . In this way, the maximum width of the second etched portion  62  is established to be larger than the width of the opening  101  of the second mask pattern  10 . The remaining insulating layer  6  may be etched by ½ of the previous thickness to thereby form the second etched portions  62 . After the formation of the second etched portions  62 , the second mask pattern  10  is removed. 
   As shown in  FIG. 1E , a third mask pattern  12  with openings  121  is formed on the insulating layer  6 . The opening  121  of the third mask pattern  12  also has the same size as that of the opening  101  of the second mask pattern  10 . For this purpose, the third mask pattern  12  is formed with a photosensitive film, which is patterned through a photolithography process using the same exposure mask as that used in forming the second mask pattern  10 . Accordingly, the third mask pattern  12  covers the sidewalls of the second etched portions  62 . 
   As shown in  FIG. 1F , the portions of the insulating layer  6  exposed through the openings  121  of the third mask pattern  12  are completely etched to thereby form third etched portions  63  exposing the cathode electrodes  4 . Even in this process, the etching is made such that under-cuts are made under the third mask pattern  12 . In this way, the maximum width of the third etched portions  63  is established to be larger than the width of the opening  121  of the third mask pattern  12 . 
   As shown in  FIG. 1G , the third mask pattern  12  is removed to thereby complete the openings  64  of the insulating layer  6 . In this way, the openings  64  are formed in the insulating layer  6  by repeating the photolithography process as well as the wet etching process three times and each completed opening  64  has an aspect ratio of more than 1. 
   It is set forth above that the formation of the openings  64  in the insulating layer  6  is made by repeating the photolithography process as well as the wet etching process three times, but it is possible that depending upon the thickness of the insulating layer  6 , the photolithography process and the etching process are conducted only two times, or four or more times. 
   As shown in  FIG. 1H , gate electrodes  14  are formed on the insulating layer  6  and electron emission regions  16  are formed within the openings  64  of the insulating layer  6 . The electron emission regions  16  are formed with a material that emits electrons when an electric field is applied thereto in a vacuum, such as a carbonaceous material or a nanometer-sized material. The electron emission regions  16  may be formed with carbon nanotube, graphite, graphite nanofiber, diamond, diamond-like carbon, C 60 , silicon nanowire or a combination thereof. 
   To form the electron emission regions  16 , an organic material such as a vehicle and/or a binder is mixed with an electron emission material to prepare a paste-phased mixture with a viscosity suitable for the printing. The mixture is screen-printed onto the target, dried and fired. Alternatively, with the formation of the electron emission regions  16 , a photosensitive material is added to the paste-phased mixture to form a new mixture and the mixture is screen-printed onto the entire surface of the first substrate and partially hardened. The non-hardened mixture is removed through developing. The electron emission regions  16  may be formed using the technique of direct growth, chemical vapor deposition or sputtering. 
   As described above, the openings  64  each with an aspect ratio of more than 1 can be easily formed in the insulating layer  6  by repeating the photolithography process and the wet etching process two or more times. With the completed opening  64 , as shown in  FIG. 2 , a first boundary portion  65  is disposed between the sidewall of the first etched portion  61  and the sidewall of the second etched portion  62  such that it is inclined against those sidewalls at a predetermined angle. A second boundary portion  66  is disposed between the sidewall of the second etched portion  62  and the sidewall of the third etched portion  63  such that it is inclined against those sidewalls at a predetermined angle. 
   As shown in  FIGS. 3 and 4 , the electron emission device includes a first substrate  2  and a second substrate  20  facing each other at a predetermined distance. An electron emission structure is provided on the first substrate  2  to emit electrons. A light emission or display structure is on the second substrate  20  to emit visible rays generated by the electrons from the electron emission structure, thereby making light emissions or displaying a desired image. 
   Cathode electrodes  4  are laid out in a stripe pattern on the first substrate  2  in one direction (in the direction of the y axis of the drawing). An insulating layer  6  is formed on the entire surface of the first substrate  2 , covering the cathode electrodes  4 . Gate electrodes  14  are patterned as stripes on the insulating layer  6  perpendicular to the cathode electrodes  4  (in the direction of the x axis of the drawing). 
   The regions where the cathode and the gate electrodes  4  and  14  cross make a formation of sub-pixel regions. Electron emission regions  16  are formed on the cathode electrodes  4  in the respective sub-pixel regions. Openings  64  and  141  are formed in the insulating layer  6  and the gate electrodes  14  corresponding to the respective electron emission regions  16 , exposing the electron emission regions  16  on the first substrate  2 . 
   The electron emission regions  16  are formed with a material that emits electrons when an electric field is applied thereto in a vacuum, such as a carbonaceous material or a nanometer-sized material. The electron emission regions  16  may be formed with carbon nanotube, graphite, graphite nanofiber, diamond, diamond-like carbon, C 60 , silicon nanowire, or a combination thereof. The formation of the electron emission regions  16  may be made using the technique of screen printing, direct growth, chemical vapor deposition or sputtering. 
   It is illustrated in the drawings that the electron emission regions  16  are circular-shaped and linearly arranged along the length of the cathode electrode  4  in the respective sub-pixel regions. However, the shape of the emission regions  16 , number of regions  16  per sub-pixel and arrangement of the electron emission regions  16  are not limited thereto, but may be altered in various manners. 
   Red, green and blue phosphor layers  22  are formed on a surface of the second substrate  20  facing the first substrate  2  at a set distance and black layers  24  are formed between the neighboring phosphor layers  22  to enhance the screen contrast. An anode electrode  26  is formed on the phosphor layers  22  and the black layers  24  with a metallic layer containing aluminum (Al). The anode electrode  26  receives a high voltage required for accelerating the electron beams from the outside and reflects the visible rays radiated from the phosphor layers  22  toward the first substrate  2  to the second substrate  20 , thereby increasing the screen luminance. 
   Alternatively, the anode electrodes may be formed with a transparent conductive film based on indium tin oxide (ITO), instead of the metallic layer. In this embodiment, the anode electrode is formed on a surface of the phosphor layers and the black layers facing the second substrate. The anode electrode may be patterned with a plurality of portions. 
   Spacers  28  are arranged between the first and the second substrates  2  and  20 . The first and the second substrates  2  and  20  are sealed to each other at their peripheries using a sealant, such as a glass frit. The inner space between the first and the second substrates  2  and  20  is evacuated to be a vacuum, thereby constructing an electron emission device. 
   The above-structured electron emission device is driven by applying predetermined voltages to the cathode electrodes  4 , the gate electrodes  14  and the anode electrode  26 . For instance, driving voltages with a voltage difference of several to several tens of volts are applied to the cathode and the gate electrodes  4  and  14 , and a positive (+) voltage of several hundreds to several thousands of volts is applied to the anode electrode  26 . 
   Electric fields are formed around the electron emission regions  16  at the pixels where the voltage difference between the cathode and the gate electrodes  4  and  14  exceeds a threshold value and electrons are emitted from those electron emission regions  16 . The emitted electrons are attracted by the high voltage applied to the anode electrode  26 , thereby colliding against the corresponding phosphor layers  22  and causing light to emit from them. 
   In the electron emission device according to the embodiments of the present invention, as the opening  64  is formed in the insulating layer  6  by the above-described method, the depth of the opening  64  is established to be larger than the width thereof and as a result, the opening  64  has a high aspect ratio of more than 1. 
   In the electron emission structure that is formed on the first substrate  2 , the micro patterning creates a large number of electron emission regions  16  that can be arranged within the limited sub-pixel area or the sub-pixel area can be minimized or reduced. In the former case, the amount of emitted electrons is increased to effectively increase the screen luminance. In the latter case, a desired high resolution can be easily achieved. 
   It will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the spirit or scope of the invention. Thus, it is intended that the present invention cover the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.