Patent Publication Number: US-2007103053-A1

Title: Image display device

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
      This is a Continuation Application of PCT Application No. PCT/JP2005/012496, filed Jul. 6, 2005, which was published under PCT Article 21(2) in Japanese.  
      This application is based upon and claims the benefit of priority from prior Japanese Patent Application No. 2004-203401, filed Jul. 9, 2004, the entire contents of which are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      This invention relates to an image display device provided with substrates located opposite each other and spacers arranged between the substrates.  
      2. Description of the Related Art  
      In recent years, various flat image display devices have been noticed as a next generation of lightweight, thin display devices to replace cathode-ray tubes (CRTs). For example, a surface-conduction electron emission device (SED) has been developed as a kind of a field emission device (FED) that functions as a flat display device.  
      The SED comprises a first substrate and a second substrate that are located opposite each other with a predetermined space between them. These substrates have their respective peripheral portions joined together by a rectangular sidewall, thereby forming a vacuum envelope. Three-color phosphor layers are formed on the inner surface of the first substrate. Arranged on the inner surface of the second substrate are a large number of electron emitting elements, which correspond to pixels, individually, and serve as electron emission sources that excite the phosphors. In order to support an atmospheric load that acts between the first substrate and the second substrate and maintain the gap between the substrates, a plurality of spacers are arranged between the two substrates. According to a device described in Jpn. Pat. Appln. KOKAI Publication No. 2002-082850, for example, a supporting substrate is provided between the first substrate and the second substrate, and the plurality of spacers are set up on the supporting substrate. The supporting substrate is formed having a plurality of electron beam apertures through which electron beams emitted individually from the electron emitting elements pass.  
      In displaying an image on the SED described above, an anode voltage is applied to the phosphor layers, and the electron beams emitted from the electron emitting elements are accelerated by the anode voltage and collided with the phosphor layers. Thereupon, the phosphors glow and display the image. In order to obtain practical display characteristics, it is necessary to use phosphors similar to those of conventional cathode ray tubes and set the anode voltage to several kV or more, and preferably, to 5 kV or more.  
      In the SED constructed in this manner, the luminance of the displayed image depends on the anode voltage, so that the anode voltage should preferably be high. In view of the resolution and the properties and manufacturability of supporting members, however, the gap between the first substrate and the second substrate is set to a relatively small value, e.g., about 1 to 2 mm. If a high voltage is applied, an intense electric field is inevitably formed in the small gap between the first substrate and the second substrate, so that electric discharge (dielectric breakdown) easily occurs between the two substrates. If the electric discharge occurs, breakdown or degradation of the electron emitting elements, a phosphor screen, or wires on the first substrate may possibly be caused. The electric discharge that results in the occurrence of such failure is not desirable in products.  
     BRIEF SUMMARY OF THE INVENTION  
      This invention has been made in consideration of these circumstances, and its object is to provide an image display device with improved reliability and display quality in which the occurrence of electric discharge is suppressed.  
      In order to achieved the object, according to an aspect of the invention, there is provided an image display device comprising: a first substrate having a phosphor screen formed thereon; a second substrate located opposite the first substrate with a gap and provided with a plurality of electron emission sources which excite the phosphor screen; a plurality of spacers which are located between the first and second substrates and support an atmospheric load acting on the first and second substrates; and a grid unit provided between the spacers and the second substrate, the grid unit including a plate-shaped grid which has a plurality of electron beam apertures opposed to the electron emission sources, individually, and is located opposite the second substrate and to which a predetermined voltage is applied, a first dielectric layer which covers an outer surface of the grid, a conductive layer provided between the first dielectric layer and the second substrate and connected to a ground potential, and a second dielectric layer formed covering the conductive layer and situated between the conductive layer and the second substrate. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING  
      The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate embodiments of the invention, and together with the general description given above and the detailed description of the embodiments given below, serve to explain the principles of the invention.  
       FIG. 1  is a perspective view showing an SED according to a first embodiment of this invention;  
       FIG. 2  is a perspective view of the SED, broken away along line II-II of  FIG. 1 ;  
       FIG. 3  is a sectional view enlargedly showing the SED;  
       FIG. 4  is a plan view showing a grid unit of the SED; and  
       FIG. 5  is a sectional view showing an SED according to a second embodiment of this invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      A first embodiment in which this invention is applied to an SED as a flat image display device will now be described in detail with reference to the drawings.  
      As shown in FIGS.  1  to  3 , the SED comprises a first substrate  11  and a second substrate  12 , which are formed of a rectangular glass plate each. These substrates are located opposite each other with a gap of about 1.0 to 2.0 mm between them. The first substrate  11  and the second substrate  12  have their respective peripheral edge portions joined together by a sidewall  13  of glass in the form of a rectangular frame, thereby forming a flat vacuum envelope  10  of which the inside is kept vacuum.  
      A phosphor screen  16  that functions as a phosphor screen is formed on the inner surface of the first substrate  11 . The phosphor screen  16  is composed of phosphor layers R, G and B, which glow red, blue, and green, individually, and light shielding layers  15  arranged side by side. These phosphor layers are stripe-shaped, dot-shaped, or rectangular. A metal back  17  of aluminum or the like and a getter film  19  are successively formed on the phosphor screen  16 .  
      Provided on the inner surface of the second substrate  12  are a large number of surface-conduction electron emitting elements  18 , which individually emit electron beams as electron sources for exciting the phosphor layers R, G and B of the phosphor screen  16 . These electron emitting elements  18  are arrayed in a plurality of columns and a plurality of rows corresponding to individual pixels. Each electron emitting element  18  is formed of an electron emitting portion (not shown), a pair of element electrodes that apply voltage to the electron emitting portion, etc. A large number of wires  21  for driving the electron emitting elements  18  are provided in a matrix on the inner surface of the second substrate  12 , and their respective end portions are led out of the vacuum envelope  10 .  
      The sidewall  13  that functions as a joint member is sealed to the peripheral edge portion of the first substrate  11  and the peripheral edge portion of the second substrate  12  with a sealant  20  of, for example, low-melting-point glass or low-melting-point metal, whereby these substrates are joined together.  
      As shown in  FIGS. 2 and 3 , the SED comprises a spacer structure  22 , which is located between the first substrate  11  and the second substrate  12 , and a grid unit  40  located between the spacer structure and the second substrate. In the present embodiment, the spacer structure  22  has a supporting substrate  24 , formed of a rectangular metal plate, and a large number of columnar spacers  30  set up integrally on one surface of the supporting substrate.  
      More specifically, the supporting substrate  24  is formed having a rectangular shape that is substantially equal in size to the phosphor screen  16 . The supporting substrate  24  has a first surface  24   a  opposed to the inner surface of the first substrate  11  and a second surface  24   b  opposed to the inner surface of the second substrate  12 , and is located parallel to these substrates. A large number of electron beam apertures  26  are formed in the supporting substrate  24  by etching or the like. The electron beam apertures  26  are arrayed opposite the electron emitting elements  18 , individually, and the electron beams emitted from the electron emitting elements pass through the respective electron beam apertures.  
      The supporting substrate  24  is formed of a plate of, for example, an iron-nickel-based metal with a thickness of 0.1 to 0.25 mm, and the electron beam apertures  26  are formed having a rectangular shape measuring 0.15 to 0.25 mm by 0.15 to 0.25 mm, for example. Formed on the surface of the supporting substrate  24  is a high-resistance film  32  as a dielectric layer obtained by spreading and firing a dielectric material that consists mainly of glass or ceramic. According to the present embodiment, the first and second surfaces  24   a  and  24   b  of the supporting substrate  24  and the respective inner wall surfaces of the electron beam apertures  26  are covered by the high-resistance film  32  of Li-based alkaline borosilicic acid glass with a thickness of about 10 μm. The supporting substrate  24  is provided in a manner such that its first surface  24   a  is in surface contact with the inner surface of the first substrate  11  with the getter film  19 , metal back  17 , and phosphor screen  16  between them.  
      If the longitudinal direction and the lateral direction of the first substrate  11  and the second substrate  12  are X and Y, respectively, the electron beam apertures  26  in the supporting substrate  24  are arrayed at a predetermined pitch in the direction X and at a pitch larger than the X-direction pitch in the direction Y. The phosphor layers R, G and B that are formed in the first substrate  11  and the electron emitting elements  18  on the second substrate  12  are arrayed at the same pitch as the electron beam apertures  26  with respect to the directions X and Y, and face the electron beam apertures, individually. Thus, the electron emitting elements  18  face their corresponding phosphor layers through the electron beam apertures  26 , individually.  
      The spacers  30  are set up integrally on the second surface  24   b  of the supporting substrate  24  and are situated between the electron beam apertures  26  that are arranged in the direction Y. The respective extended ends of the spacers  30  abut the grid unit  40 , which will be mentioned later. Each of the spacers  30  is tapered so that its diameter is reduced from the side of the supporting substrate  24  toward its extended end. The cross section of each spacer  30  in a direction parallel to the grid surface is substantially elliptic.  
      The spacer structure  22  constructed in this manner is located between the first substrate  11  and the second substrate  12 . In the spacer structure  22 , moreover, the supporting substrate  24  is in surface contact with the first substrate  11 , and the respective extended ends of the spacers  30  abut the inner surface of the second substrate  12  with interposing the grid unit  40  therebetween, thereby supporting an atmospheric load that acts on these substrates and keeping the space between the substrates at a predetermined value.  
      As shown in FIGS.  2  to  4 , the grid unit  40  has a grid  42  in the form of a rectangular plate that is substantially equal in size to the phosphor screen  16 . The grid  42  has two surfaces opposed to the inner surface of the first substrate  11  and the inner surface of the second substrate  12  and is located parallel to these substrates. A large number of electron beam apertures  44  are formed in the grid  42  by etching or the like. The electron beam apertures  44  are arrayed at a predetermined pitch in the direction X and at a pitch larger than the X-direction pitch in the direction Y. The electron beam apertures  44  are arrayed opposite the electron emitting elements  18 , individually, and the electron beams emitted from the electron emitting elements pass through the electron beam apertures  44 , respectively.  
      The grid  42  is formed a plate of, for example, an iron-nickel-based metal with a thickness of 0.1 to 0.25 mm, and the electron beam apertures  44  are rectangular. The surface of the grid  42  including the respective inner surfaces of the electron beam apertures  44  is covered by a first dielectric layer  46  with a thickness of about 10 μm. The first dielectric layer  46  is formed by spreading and firing a dielectric material that consists mainly of glass or ceramic, e.g., Li-based alkaline borosilicic acid glass.  
      Conductive layers  48  of a metal, such as aluminum, copper, or silver, are formed covering the first dielectric layer  46  on that surface of the grid  42  on the side of the second substrate  12 . The conductive layers  48  are formed over the whole surface of the grid  42  except the electron beam apertures  44 . In the present embodiment, the conductive layers  48  are in the form of striped-shaped conductive layers that individually extend in the direction X and are situated between the electron beam apertures  44  that are arranged side by side in the direction Y.  
      A second dielectric layer  50  is formed covering the conductive layers  48  on that surface of the grid  42  on the side of the second substrate  12 . The second dielectric layer  50  is formed by spreading and firing a dielectric material that consists mainly of glass or ceramic, e.g., Li-based alkaline borosilicic acid glass.  
      Another set of conductive layers  52  of a metal, such as aluminum, copper, or silver, are formed covering the first dielectric layer  46  on that surface of the grid  42  on the side of the first substrate  11 . The conductive layers  52  are formed over the whole one surface of the grid  42  except the electron beam apertures  44 . In the present embodiment, the conductive layers  52  are in the form of striped-shaped conductive layers that individually extend in the direction X and are situated between the electron beam apertures that are arranged side by side in the direction Y. The conductive layers  52  and the conductive layers  48  are formed by screen printing, vapor deposition, sputtering, CVD, etc.  
      A third dielectric layer  54  is formed covering the conductive layers  52  on that surface of the grid  42  on the side of the first substrate  11 . The third dielectric layer  54  is formed by spreading and firing a dielectric material that consists mainly of glass or ceramic, e.g., Li-based alkaline borosilicic acid glass.  
      The grid unit  40  constructed in this manner is provided on the second substrate  12  with the second dielectric layer  50  in contact with the inner surface of the second substrate. The electron beam apertures  44  of the grid unit  40  individually face their corresponding electron emitting elements  18 . In the present embodiment, the grid unit  40  is located overlapping the wires  21  that are formed on the second substrate. Thus, a slight gap, e.g., a gap of about 20 μm, is defined between the dielectric layer  50  and the inner surface of the second substrate  12 . This gap is formed so as to account for 50% or less of the diameter of each electron beam aperture  44 . Further, the wires  21  function as gap defining members that define gaps between the grid unit  40  and the second substrate  12 .  
      The plurality of spacers  30  that constitute the spacer structure  22  abut the third dielectric layer  54  of the grid unit  40  in regions between the electron beam apertures  44 , individually. Thus, the grid unit  40  is held between the spacers  30  and the second substrate  12 .  
      The SED comprises a voltage supply portion that applies voltage to the grid unit  40  and the metal back  17  of the first substrate  11 . The voltage supply portion has a first power source  60   a  that applies a high voltage of, e.g., about 8 kV to the metal back  17  and a second power source  60   b  that applies a voltage of, e.g., about 1 kV to the conductive layers  52 . The second substrate  12  and the conductive layers  48  that are situated between the grid  42  and the second substrate  12  are connected to the ground potential.  
      In displaying an image on the SED constructed in this manner, the electron beams emitted from the electron emitting elements  18  are accelerated by an anode voltage that is applied to the phosphor screen  16  and the metal back  17  and collided with the phosphor screen  16 . Thereupon, the phosphor layers of the phosphor screen  16  are excited to luminescence and display the image. As this is done, the grid  42  to which the voltage is applied functions as an extraction electrode for extracting the electron beams from the electron emitting elements  18 . The conductive layers  52  on the side of the first substrate  11  to which the voltage is applied have a function to converge the electron beams transmitted through the electron beam apertures  44  toward the phosphor layers.  
      According to the SED constructed in this manner, the grid unit  40  that has the grid  42  and the conductive layers  48  is provided on the inner surface of the second substrate  12 , a predetermined voltage is applied to the grid  42 , and the conductive layers  48  that are situated between the grid and the second substrate are connected to the ground potential. Accordingly, the grid unit  40  can substantially reduce the intensity of an electric field that is generated on the inner surface of the second substrate  12  to zero, i.e., 0 V/m, thereby suppressing the occurrence of electric discharge (creeping discharge). Thus, an SED with improved reliability and display quality can be provided.  
      The grid  42  is provided near the electron emitting elements  18  and functions also as an extraction electrode. Therefore, the electron beams can be emitted efficiently. Further, the grid unit  40  has the other conductive layers  52  that are provided on the side of the first substrate  11 , and the convergence of the electron beams on the phosphor layers can be improved by applying voltage to the conductive layers. Based on these circumstances, an SED with further improved display quality can be obtained.  
      The following is a description of a second embodiment of the invention. According to the second embodiment, as shown in  FIG. 5 , a grid unit  40  has a grid  42 , a first dielectric layer  46 , conductive layers  48 , and a second dielectric layer  50 . Conductive layers and a third dielectric layer on the side of a first substrate  1  are omitted. The grid  42  is provided over a second substrate  12  with the first dielectric layer  46 , conductive layers  48 , and second dielectric layer  50  between them. The grid  42  is connected to a second power source  60   b , while the conductive layers  48  are connected to the ground potential.  
      A plurality of spacers  30  are provided in place of the aforementioned spacer structure between the grid unit  40  and the first substrate  11 . These spacers  30  are columnar or plate-shaped. One end of each spacer  30  abuts the first dielectric layer  46  of the grid unit  40  between adjacent electron beam apertures  44 , and the other end thereof abuts the inner surface of the first substrate  11  with interposing a getter film  19 , a metal back  17 , and light shielding layers  15  between them. Thus, the spacers  30  support an atmospheric load that acts on the first substrate  11  and the second substrate  12 , thereby keeping the space between the substrates at a predetermined value.  
      In the second embodiment, other configurations are the same as those of the foregoing first embodiment, so that like reference numerals are used to designate like portions, and a detailed description thereof is omitted.  
      According to the SED constructed in this manner, the grid unit  40  that has the grid  42  and the conductive layers  48  is provided on the inner surface of the second substrate  12 , a predetermined voltage is applied to the grid  42 , and the conductive layers  48  that are situated between the grid and the second substrate are connected to the ground potential. Accordingly, the grid unit  40  can reduce the intensity of an electric field that is generated on the inner surface of the second substrate  12 , thereby suppressing the occurrence of electric discharge. The grid  42  is provided near the electron emitting elements  18  and functions also as an extraction electrode. Thus, an SED with improved reliability and display quality can be provided.  
      The present invention is not limited directly to the embodiments described above, and its components may be embodied in modified forms without departing from the spirit of the invention. Further, various inventions may be made by suitably combining a plurality of components described in connection with the foregoing embodiments. For example, some of the components according to the embodiments may be omitted. Furthermore, components according to different embodiments may be combined as required.  
      In the foregoing embodiments, the gap defining members that define the gaps between the second substrate and the grid unit are formed of the wires on the second substrate. Alternatively, however, they may be formed of a plurality of independent spacers.  
      The diameter and height of the spacers, the dimensions and materials of the other components, the voltage applied to the grid, etc. are not limited to the foregoing embodiments, but may be suitably selected as required. This invention is not limited to image display devices that use surface-conduction electron emitting elements as electron sources, but may be also applied to image display devices that use other electron sources, such as the field-emission type, carbon nanotubes, etc.