Patent Publication Number: US-2004046755-A1

Title: Display units and their fabrication methods

Description:
BACKGROUND OF THE INVENTION  
       [0001] 1. Field of the Invention  
       [0002] The present invention relates to display units and their fabrication methods. More specifically, the present invention relates to display units having a function which uses nanotube electron emitters as electron emitters to illuminate phosphor layers, displaying image information on a panel and their fabrication methods.  
       [0003] 2. Description of the Related Art  
       [0004] Nanotube electron emitters as electron emitters having carbon, boron and nitrogen as constituents are known. A carbon nanotube electron emitter having carbon as a constituent will be described here as a representative example.  
       [0005] There have been reported many carbon nanotube electron emitters and emissive type flat-panel display units using the same as electron emitters. An example in which a 4.5-inch emissive type flat-panel display unit is fabricated is described in SID 99 Digest pp. 1134-1137. The emissive type of the emissive type flat-panel display unit illuminates phosphor layers provided on an image display panel by irradiating an excitation light such as an electron beam or an ultraviolet light to display an image. It is distinguished from an LCD (Liquid Crystal Display) which is not emissive.  
       [0006] In the prior art method described in the document, a paste for forming carbon nanotube electron emitters includes a glass-constituent as an adhesive.  
       [0007] The glass constituent as an adhesive remains as glass of an electrical insulation material when the paste is heat treated. The percentage of the electrically connected carbon nanotubes is at most several ten % at a micro-level. The emitting point density is below 1000 points/cm 2 . The emitting in-plane uniformity is very low.  
       [0008] The low emitting point density means that the electron beam emitting density of the electron emitters for exciting the phosphor layers is low, resulting in nonuniformity. The screen is dark and the displayed image is not uniform, thereby deteriorating the image quality significantly. The problem will be serious as the image display panel is larger to increase the displayed area.  
       SUMMARY OF THE INVENTION  
       [0009] To solve the above prior art problems, an object of the present invention is to provide display units which can increase the emitting point density of nanotube electron emitters as electron emitters and have a good image quality and their fabrication methods.  
       [0010] To achieve the above object, the present inventors have experimented and studied various fabrication methods which can increase the characteristic of nanotube electron emitters as electron emitters of display units and can easily obtain-electron emitters having a high reliability. We have obtained findings that high-performance nanotube electron emitters can be obtained by industrially easy fabrication methods to realize excellent display units.  
       [0011] The present invention has been made based on such important findings. In summary, a low melting point metal material, not glass, is used as an adhesive in a paste containing nanotubes to secure complete electric conduction at a micro-level, thereby reliably securing electric conduction of the nanotubes and the electrode base material.  
       [0012] This can increase the emitting point density above 100000 points/cm 2 . An in-plane uniform emitting pattern can be realized.  
       [0013] The nanotubes targeted by the present invention are single-wall nanotubes of a single-layer tubular structure composed of at least one of elements of carbon, boron and nitrogen or multiwall nanotubes of a nesting-like multilayer tubular structure. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0014]FIG. 1 is an explanatory view of Embodiment 1 of the present invention;  
     [0015]FIG. 2 is an explanatory view of Embodiment 2 of the present invention;  
     [0016]FIG. 3 is an explanatory view of Embodiment 2 of the present invention;  
     [0017]FIG. 4 is an explanatory view of Embodiment 2 of the present invention;  
     [0018]FIG. 5 is an explanatory view of Embodiment 2 of the present invention;  
     [0019]FIG. 6 is an explanatory view of Embodiment 2 of the present invention;  
     [0020]FIG. 7 is an explanatory view of Embodiment 2 of the present invention;  
     [0021]FIG. 8 is an explanatory view of Embodiment 2 of the present invention;  
     [0022]FIG. 9 is an explanatory view of Embodiment 2 of the present invention;  
     [0023]FIG. 10 is an explanatory view of Embodiment 3 of the present invention;  
     [0024]FIG. 11 is an explanatory view of Embodiment 3 of the present invention;  
     [0025]FIG. 12 is an explanatory view of Embodiment 3 of the present invention;  
     [0026]FIG. 13 is an explanatory view of Embodiment 3 of the present invention; and  
     [0027]FIG. 14 is an explanatory view of Embodiment 3 of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     [0028] The features of the present invention will be described below more specifically.  
     [0029] In a first invention of the present invention, a display unit having:  
     [0030] an electron emitter substrate having nanotube electron emitters as electron emitters, the electron emitters being formed in a matrix form in the crossing parts of scan lines and signal lines;  
     [0031] an image display panel having a phosphor screen having phosphor layers and an anode electrode arranged by forming a space at a predetermined pitch opposite the electron emitter substrate; and  
     [0032] a control part transmitting image information to the image display panel to display an image, wherein  
     [0033] in the nanotube electron emitters forming electron emitters, nanotubes and granular support media composed of an electric conductor are mixed with each other, at least one end of the nanotubes and the support media are adhered onto the substrate by melted metal adhesives, and the other end of the nanotubes is oriented as a free end in the vertical direction to the substrate by the support action of the support media.  
     [0034] In a second invention, the granular support media are composed of a granular electric conductor not dissolved at the melting adhesion temperature of the metal adhesives. As the preferable granular electric conductor, at least one granular electric conductor selected from the group of C, Ag, Au, Pt, Pd, Ni, Fe, Cu and Co is given.  
     [0035] In a third invention, the metal adhesives include at least one metal selected from the metal group of Sn, Pb, Bi, In, Cd, Zn, Ag and Al.  
     [0036] In a fourth invention, the nanotubes include single-wall nanotubes of a single-layer tubular structure composed of at least one element of carbon, boron and nitrogen. Preferably, the single-wall nanotubes have an average length of 0.5 to 2.0 microns.  
     [0037] In a fifth invention, the nanotubes include multiwall nanotubes of a nesting-like multilayer tubular structure composed of at least one element of carbon, boron and nitrogen.  
     [0038] In a sixth invention, the single-wall nanotubes are nanotubes having an average length of 0.5 to 2.0 microns. Preferably, the multiwall nanotubes have an average length of 0.5 to 5.0 microns.  
     [0039] In a seventh invention, a fabrication method of the display unit having:  
     [0040] a step of fabricating an electron emitter substrate having nanotube electron emitters as electron emitters, the electron emitters being formed in a matrix form in the crossing parts of scan lines and signal lines;  
     [0041] a step of fabricating a phosphor screen having phosphor layers and an anode electrode arranged by forming a space at a predetermined pitch opposite the electron emitter substrate; and  
     [0042] an assembling step for fixing the electron emitter substrate and the phosphor screen via a frame, wherein  
     [0043] the step of fabricating nanotube electron emitters forming electron emitters includes the steps of: preparing a paste including nanotubes, granular support media composed of an electric conductor, metal adhesives, and organic compounds for pasting; forming a nanotube electron emitter pattern by printing or coating the paste onto the substrate; and heat treating the same.  
     [0044] Embodiments  
     [0045] Embodiments of the present invention will be described below specifically according to the drawings.  
     [0046] &lt;Embodiment 1&gt; 
     [0047] Embodiment 1 of the present invention will be described using FIG. 1 and Table 1. FIG. 1( a ) schematically shows the state of a nanotube paste printed or coated onto a glass substrate  101 . The nanotube paste includes nanotubes  102 , support media  103 , metal adhesives  104  and organic compounds  105 .  
     [0048] The nanotubes  102  are used as electron emitters. The nanotubes have a diameter of about 0.7 to 50 nm and a length of 0.5 to several ten microns. Because of their very long and narrow structures, an electric field concentrates on their edges. It is possible to obtain an emitting current density enough to realize an emissive type flat-panel display unit of several ten mA/cm 2  with a very low electric field of several V/micron.  
     [0049] A single-layer nanotube is called a single-wall nanotube. A multilayer nanotube in which single walls are of a concentric nesting structure is called a multiwall nanotube.  
     [0050] The present invention can use either the single-wall nanotube and the multiwall nanotube. The present invention can also use a composite thereof. A nanotube composed of a carbon atom is called a carbon nanotube. Other than the carbon nanotube, a nanotube composed of boron and nitrogen elements is also known.  
     [0051] A nanotube can be composed of three elements of carbon, boron and nitrogen. The present invention can also use a nanotube composed of every element.  
     [0052] The support media  103  are made of a granular material as an electric conductor and are used for orienting the nenotubes  102  in the vertical direction to the substrate  101 . When the later-described metal adhesives  104  are melted to adhere the nanotube  102 , the support media  103  must hold their granular shape without being dissolved. When the length of the nanotubes  102  is about 1 micron, the size of the support media  103  is also desirably about 1 micron.  
     [0053] Desirable is the material of the support media  103  which is hard to form an oxide on the surface. Otherwise, desirable is the material of the support media  103  in which an oxide is conductive. It is possible to use a metal such as Ag, Au, Pt, Pd, Ni, Fe, Cu and Co or an alloy thereof. It is also possible to use graphite and spherical graphite.  
     [0054] When the nanotube paste is heat treated, the metal adhesives  104  adhere the nanotubes  102  and the support media  103  onto the substrate  101  and are used to secure electric conduction of the nanotubes  102  and the support media  103 . Low melting point metal particles can be used as the metal adhesives  104 . Examples of low melting point metals and alloys thereof are shown in Table 1.  
                           TABLE 1                                      Melting               temperature                                                     No.   (° C.)   Sn   Pb   Bi   In   Cd   Zn   Ag   Al                                                             1   57.8   12   18   49   21                       2   78.9   17       57   26       3   95   15.5   32   52.5       4   100   22   28   50       5   134.2   37.5   37.5       25       6   182       50       50       7   183   61.9   38.1           8   183   63   37           9   183   60   40           10   183   55   45           11   183   50   50           12   183   45   55           13   183   40   60           14   176   25               75       15   266                   82.5   17.5       16   300   5   95           17   304       97.5                   2.5       18   419                       100       19   382                       95       5       20   200   91                   9       21   200   70                   30       22   200   60                   40       23   200   30                   70       24   265                   10   90       25   265                   40   60       26   171   34   63               3                  
 
     [0055] Table 1 shows compositions of the metals and their melting temperatures. It is possible to use a metal such as Sn, Pb, Bi, In, Cd, Zn, Ag and Al and an alloy thereof.  
     [0056] The organic compounds  105  are used as a solvent for pasting. In consideration of printability or coatability, various organic compounds  105  can be used.  
     [0057] As an example of the nanotube paste composition, the nanotube paste is prepared using multiwall nanotubes having an average diameter of 20 nm and an average length of 1 micron as the nanotubes  102 , silver fine grains having an average diameter of 1 micron as the support media  103 , zinc particles having an average diameter of 0.1 micron as the metal adhesives  104 , and a mixture of terpineol and ethyl cellulose as the organic compounds  105 .  
     [0058]FIG. 1( b ) schematically shows the state of the heat-treated nanotube paste. The organic compounds  105  are fired and disappear by a heat treatment at 450° C. for 30 minutes. The metal adhesives  104  are melted by the heat treatment and adhere the support media  103  and the nanotubes  102  onto the substrate to secure electric conduction of the support media  103  and the nanotubes  102 .  
     [0059] An electric field is applied to the electron emitters fabricated on the glass substrate  101  to irradiate the emission electrons onto the opposite phosphor screen. The emitting pattern is then observed. The very uniform emitting pattern can be obtained. When it is observed at a micro-level, the emitting point density is above 100000 points/cm 2 . The emitting point density can be increased by above double figures as compared with the prior art electron emitters formed by a paste using glass adhesives.  
     [0060] &lt;Embodiment 2&gt; 
     [0061] Embodiment 2 of the present invention will be described using FIGS. 2, 3,  4 ,  5  and  6  to  9 .  
     [0062] Using the disassembling diagram of FIG. 2, the entire structure of an emissive type flat-panel display unit (image display panel) of the present invention will be described. FIG. 2( a ) shows a perspective view looking down from slantingly above. FIG. 2( b ) shows a perspective view looking up from slantingly below. The emissive type flat-panel display unit has an electron emitter substrate  301  in which electron emitter arrays are fabricated, a phosphor screen  303  in which phosphor stripes or dots are fabricated corresponding to the positions of electron emitters, and a frame glass  302  for fixing the electron emitter substrate  301  and the phosphor screen  303  at a predetermined pitch.  
     [0063] Although not shown, as the screen size is increased, the frame glass needs in its inside spacers for holding the electron emitter substrate  301  and the phosphor screen  303  at a predetermined pitch.  
     [0064] Using FIG. 3, the structure of the electron emitter substrate  301  will be described. A plurality of cathode electrode stripes  401  are formed in the horizontal direction. A plurality of gate electrode stripes  402  are formed in the vertical direction. The cathode electrode stripes  401  and the gate electrode stripes  402  cross each other by interposing a dielectric layer  605 . An electron emitter  403  is formed at each of the crossing points.  
     [0065]FIG. 3( a ) shows a plan view. FIG. 3( b ) shows a partially enlarged view of the electron emitter  403  formed at the crossing point of the cathode electrode stripe  401  and the gate electrode stripe  402 . FIG. 3( c ) shows a partially enlarged view taken along line A-A′ of FIG. 3( b ).  
     [0066] The electron emitter  403  is formed on the surface of the cathode electrode stripe  401  in the bottom part of an electron emitter hole  403   a  through the gate electrode stripe  402  and the dielectric layer  605  thereunder. The electron emitter  403  using the nanotube is formed by the method according to Embodiment 1 as described later.  
     [0067] Using FIG. 4, the structure of the phosphor screen  303  will be described. FIG. 4( a ) is a plan view. FIG. 4( b ) is a partially enlarged view. Corresponding to the positions of the electron emitters  403 , red phosphor stripes  501 , green phosphor stripes  502  and blue phosphor stripes  503  are formed.  
     [0068] Corresponding to the horizontal pitch of the electron emitters  403  provided on the electron emitter substrate  301 , black matrix stripes are fabricated by a lift-off method in regions corresponding to the center position between the electron emitters. A repeated stripe pattern of the red phosphor stripes  501 , the green phosphor stripes  502  and the blue phosphor stripes  503  is formed by a slurry method.  
     [0069] Each of the phosphor stripes is arranged in the center of the black stripes at both sides. Although not shown, after fabricating the phosphor stripes, aluminum of 50 nm is deposited onto the entire surface to form an anode electrode.  
     [0070] The thus fabricated electron emitter substrate  301  and phosphor screen  303  are arranged to be opposite at a fixed pitch using the frame glass  302 . After matching the positions of the electron emitters and the phosphor stripes, the display unit (image display panel) is completed by vacuum sealing its inside (see FIG. 3).  
     [0071] A scan signal is applied to the cathode electrode stripes  401 . An image signal is applied to the gate electrode stripes  402 . A plus accelerating voltage is applied to the anode electrode (not shown) of the phosphor screen  303  and the cathode electrodes  401  to display an image which is illuminated uniformly.  
     [0072] The detailed structure on the electron emitter substrate  301  will be described using FIG. 5. FIG. 5( a ) is a top view. FIG. 5( b ) is a cross-sectional view taken along line A-A′. FIG. 5( c ) is a cross-sectional view taken along line B-B′.  
     [0073] First,  600  cathode electrode stripes  401  having a thickness of 0.2 to 10 um, a width of 300 um and a pitch of 60 um are formed on the surface of the glass substrate  101 . The dielectric layer  605  is then formed. The dielectric layer  605  is obtained after, as described later, printing a photosensitive dielectric paste to form and fire the electron emitter holes  403   a  by a photolithography process.  
     [0074] The dielectric layer  605  has a thickness of 1 to 50 um and has the electron emitter holes  403   a  having a diameter of 1 to 50 um holed in the crossing parts of the cathode electrode stripes  401  and the gate electrode stripes  402 . After firing the dielectric layer  605 , 2400 gate electrode stripes  402  having a thickness of 0.2 to 10 um, a width of 90 um and a pitch of 30 um are formed thereon.  
     [0075] The gate electrode stripes  402  have the same electron source holes  403   a  as those of the dielectric layer  605  holed in the crossing parts of the cathode electrode stripes  401  and the gate electrode stripes  402 .  
     [0076] Using the thus fabricated wiring structure, a scan signal is inputted to the cathode electrode stripes  401  and an image signal is inputted to the gate electrode stripes  402 . An accelerating voltage is applied between the cathode electrode stripes  401  and the anode electrode, not shown, provided on the phosphor screen  303  of FIG. 4. An image which is illuminated uniformly can be displayed.  
     [0077] The detail of the fabrication process of the electron emitter substrate  301  will be described according to FIGS.  6  to  9 . As shown in FIG. 6( a ),  600  cathode electrode stripes  401  having a width of 300 um and a pitch of 60 um are formed on the glass substrate  101 . The cathode electrode stripes  401  are formed by screen printing the paste shown in Embodiment 1. Their thickness is 1 um. FIG. 6( b ) shows a cross-sectional view taken along line A-A′ of FIG. 6( a ).  
     [0078] As shown in FIG. 7( a ), a photosensitive dielectric paste  705  is screen printed on the entire surface to form the electron emitter holes  403   a  by a typical photolithography process. The same is fired in an atmosphere at 550° C. for 30 minutes to form the dielectric layer  605 . The thickness of the dielectric layer  605  is 10 um.  
     [0079] As shown in FIG. 8( a ), a photosensitive Ag paste  702  is screen printed on the entire surface. FIG. 8( b ) shows a cross-sectional view taken along line A-A′ of FIG. 8( a ).  
     [0080] As shown in FIG. 9( a ), the gate electrode stripes  402  are formed by the typical photolithography method and are fired in an atmosphere at 500° C. for 30 minutes. FIG. 9( b ) shows a cross-sectional view taken along line A-A′ of FIG. 9( a ). FIG. 9( c ) shows a cross-sectional view taken along line B-B′ of FIG. 9( a ). 2400 gate electrode stripes  402  having a width of 90 um and a pitch of 30 um are formed. The thickness of the gate electrode stripes is 5 um. The hole structures of the same size or slightly larger are formed in the same parts as those of the dielectric layer  605 .  
     [0081] The nanotube paste is filled into the electron emitter holes  403   a  of the electron emitter substrate  301  formed with the cathode electrode stripes  401 , the dielectric layer  605 , and the gate electrode stripes  402  by a printing method to form the electrode emitters  403  by the fabrication method according to Embodiment 1.  
     [0082] &lt;Embodiment 3&gt; 
     [0083] Embodiment 3 of the present invention will be described according to FIGS. 10 and 11 to  14 . The structure on the electron emitter substrate  301  of this embodiment is different from that of Embodiment 2. The structure of the electron emitter substrate  301  will be described according to FIG. 10.  
     [0084]FIG. 10( a ) is a top view. FIG. 10( b ) is a cross-sectional view taken along line A-A′ of FIG. 10( a ). FIG. 10( c ) is a cross-sectional view taken along line B-B′ of FIG. 10( a ). 600 cathode electrode stripes  401  having a thickness of 0.2 to 10 um, a width of 300 um and a pitch of 60 um are formed on the surface of the glass substrate  101 .  
     [0085] The dielectric layer  605  is then formed. The dielectric layer  605  has a thickness of 1 to 50 um and has the electron emitter holes  403   a  having a diameter of 1 to 50 um holed in the crossing parts of the cathode electrode stripes  401  and the gate electrode stripes  402 .  
     [0086] After firing the dielectric layer  605 , 2400 gate electrode stripes  402  having a thickness of 0.2 to 10 um, a width of 90 um and a pitch of 30 um are formed thereon. The gate electrode stripes  402  have the same electron emitter holes  403   a  as those of the dielectric layer  605  holed in the crossing parts of the cathode electrode stripes  401  and the gate electrode stripes  402 .  
     [0087] Finally, the electron emitters  403  are formed in the bottom part of the electron emitter holes  403   a  by the same method as that of Embodiment 2.  
     [0088] Using the thus fabricated wiring structure, a scan signal is inputted to the cathode electrode stripes  401  and an image signal is inputted to the gate electrode stripes  402 . An accelerating voltage is applied between the cathode electrode stripes  401  and the anode electrode, not shown, provided on the phosphor screen  303  of FIG. 4. An image which is illuminated uniformly can be displayed.  
     [0089] The detail of the fabrication process of the electron emitter substrate  301  will be described using FIGS.  11  to  14 . As shown in FIG. 11( a ), 600 cathode electrode stripes  401  having a width of 300 um and a pitch of 60 um are formed on the glass substrate  101 . FIG. 11( b ) shows a cross-sectional view taken along line A-A′ of FIG. 11( a ). The material of the cathode electrode stripes  401  is Ag and its thickness is 1 um.  
     [0090] As shown in FIG. 12( a ), the photosensitive dielectric paste  705  is screen printed on the entire surface to form the electron emitter holes  403   a  by the typical photolithography process. The same is fired in an atmosphere at 550° C. for 30 minutes to form the dielectric layer  605 . The thickness of the dielectric layer  605  is 10 um.  
     [0091] As shown in FIG. 13( a ), the photosensitive Ag paste  702  is screen printed on the entire surface. FIG. 13( b ) shows a cross-sectional view taken along line A-A′ of FIG. 13( a ). As shown in FIG. 14( a ), the gate electrode stripes  402  are formed by the typical photolithography method and are fired in an atmosphere at 500° C. for 30 minutes. FIG. 14( b ) is a cross-sectional view taken along line A-A′ of FIG. 14( a ). FIG. 14( c ) is a cross sectional view taken along line B-B′ of FIG. 14( a ).  
     [0092] 2400 gate electrode stripes  402  having a width of 90 um and a pitch of 30 um are formed. The thickness of the gate electrode stripes is 5 um. The hole structures  403   a  of the same size or slightly larger is formed in the same parts as those of the dielectric layer  605 .  
     [0093] Finally, the electron emitters  403  are formed in the bottom part of the electron emitter holes  403   a  by coating the nanotube paste shown in Embodiment 1 using an ink jet method.  
     [0094] In this embodiment, the cathode electrode stripes  401  and the gate electrode stripes  402  are formed by a specific metal. Any metal having required electric conduction can be used. An alloy and a metal multilayer film can be also used.  
     [0095] There is used the method for coating the carbon nanotube onto desired positions by the ink jet method. The carbon nanotube can be also arranged in the bottom part of the electron emitter holes  403   a  by any other method.  
     [0096] As described above in detail, the present invention can achieve the desired object to realize display units which can increase the emitting point density of the nanotube electron emitters as electron emitters and have a good image quality and their fabrication methods. Specifically, the emitting point density can be above 100000 points/cm 2 . An in-plane uniform emitting pattern enough to realize the emissive type flat-panel display units can be realized.