Patent Publication Number: US-2007103051-A1

Title: Image display apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
      This is a Continuation Application of PCT Application No. PCT/JP2005/010179, filed Jun. 2, 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. 2004-170039, filed Jun. 8, 2004, the entire contents of which are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      This present invention relates to a flat shaped image display apparatus including a pair of substrates which are opposed to each other and a frame member arranged between the substrates.  
      2. Description of the Related Art  
      In recent years, various image display devices have been developed as next-generation light-weight, small-thickness display devices, which will take the place of cathode-ray tubes (hereinafter, referred to as CRTs). Such image display devices include liquid crystal displays (LCDs) which control the intensity of light by making use of alignment of liquid crystal, plasma display panels (PDPs) which cause phosphors to emit light by ultraviolet of plasma discharge, field emission displays (FEDs) which cause phosphors to emit light by electron beams of field-emission-type electron emitting elements, and further, as one of the FED, a flat display device surface-conduction electron-emitter displays (SEDs) which cause phosphors to emit light by electron beams of surface-conduction-type electron emitting elements.  
      The FED, for example, generally comprises a front substrate and a rear substrate that are opposed to each other across a predetermined gap. These substrates have their respective peripheral portions joined together by a rectangular frame, thereby forming a vacuum envelope. A phosphor screen is formed on the inner surface of the front substrate. Provided on the inner surface of the rear substrate are a large number of electron emitting elements for use as electron emission sources, which excite the phosphors to luminescence.  
      A plurality of support members are provided between the rear substrate and the front substrate in order to support an atmospheric-pressure load acting on these substrates. The rear substrate-side potential is substantially set at a ground potential, and an anode voltage is applied to the phosphor surface. Electron beams, which are emitted from a number of the electron emitting elements, are applied to red, green and blue phosphors of the phosphor screen, and cause the phosphors to emit light. Thereby, an image is displayed.  
      According to the display apparatus constructed in this manner, the thickness of the display device can be reduced to about several millimeters, so that the device can be made lighter in weight and thinner than CRTs that are used as displays of existing TVs or computers.  
      In the above-described FED, it is necessary to maintain the interior of the envelope in a high vacuum state. Jpn. Pat. Appln. KOKAI Publication No. 2001-229825, for example, has proposed a method, as means for evacuating an envelope, for performing, in a vacuum vessel, final assembly of a front substrate and rear substrate that form the envelope. In this method, a low-melting-point metal material, such as In, suitable for a simultaneous process of attaching and sealing, is used as a sealing material. The method enables the attaching/sealing process and vacuum-sealing process to be performed simultaneously. Further, it does not require such a long process time as the time required when the envelope is exhausted using an exhaust pipe. Among other advantages, it can provide an extremely high degree of vacuum.  
      However, the frame of glass is expensive, and the method includes a process for bonding the frame to one of the substrates in the atmosphere. Therefore, the method involves a high manufacturing cost. As means for overcoming the problem, Jpn. Pat. Appln. KOKAI Publication No. 2003-068238, for example, has proposed a method for performing, in a vacuum atmosphere, bonding of substrates to a metal frame. In this method, current is passed through In to melt it for adhesion.  
      When the substrates and frame are joined in a sealed state in a vacuum, if the frame is deformed even only a little, molten In moves along the deformed frame during baking in a vacuum, with the result that variations may well occur in the height or thickness of In. If, in this state, current is passed through the frame to execute heating adhesion, In cannot simultaneously be melted over the entire region. Accordingly, a long time is required to melt In over the entire region, which inevitably results in a reduction in mass productivity. Further, local heating of In may occur. In this case, the wettability of In may be degraded because of excessive local heat generation, whereby atmospheric ingress due to defective adhesion, or breakage of the substrates due to an increase in thermal deformation stress may occur.  
      When in the adhesion process, the frame is put in a vacuum, the frame pressed by the upper and lower substrates via In of a solid state assumes a free state after the solid In is melted by heating due to power distribution. In this state, movement of In occurs as in the above case, and hence the same problem as the above occurs. To avoid this, a technique for highly accurately processing the frame for a large display in accordance with the adhesion position is necessary. Since frames are thin and are not so rigid, even a frame as designed cannot be easily handled, and may well be deformed during transfer to a vacuum vessel for substrate adhesion.  
     BRIEF SUMMARY OF THE INVENTION  
      The present invention has been developed in light of the above, and aims to provide a flat image display apparatus of high airtightness suitable for mass production.  
      In order to achieve the object, there is provided an image display apparatus which comprises a front substrate and a rear substrate opposing each other with a gap defined therebetween; and a frame arranged between peripheral edges of the front and rear substrates and attached in a sealed state to the front and rear substrates by a sealing material, the frame being formed by bonding a plurality of frame members, and includes at least two joints. 
    
    
     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 partly broken perspective view illustrating an FED according to an embodiment of the invention;  
       FIG. 2  is a sectional view taken along line II-II;  
       FIG. 3  is a plan view illustrating the phosphor screen of the FED;  
       FIG. 4  is a plan view illustrating the front substrate and frame of the FED;  
       FIG. 5  is a sectional view illustrating a state in which a rear substrate and the front substrate with the frame are opposed to each other immediately before they are put into a vacuum apparatus;  
       FIG. 6  is a view schematically illustrating a vacuum processing apparatus used for producing the FED;  
       FIG. 7  is a plan view illustrating the front substrate and frame of an FED according to a first modification of the invention;  
       FIG. 8  is a plan view illustrating the front substrate and frame of an FED according to a second modification of the invention;  
       FIG. 9  is a plan view illustrating the front substrate and frame of an FED according to third modification of the invention; and  
       FIG. 10  is a plan view illustrating the front substrate and frame of an FED according to a fourth modification of the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      An FED as an image display apparatus according to an embodiment of the invention will be described in detail with reference to the accompanying drawings.  
      As shown in  FIGS. 1 and 2 , the FED comprises, as insulated substrates, a front substrate  11  and rear substrate  12  that are formed of rectangular glass and oppose each other with a gap of about 1.5 to 3.0 mm therebetween. The front substrate  11  and rear substrate  12  have their peripheral edges bonded to each other by a rectangular frame  13  functioning as a side wall, thereby providing a flat, rectangular vacuum envelope  10  having its interior maintained in a vacuum state.  
      The vacuum envelope  10  contains a plurality of plate-shaped support members  14  for supporting the atmospheric pressure applied to the front and rear substrates  11  and  12 . The support members  14  extend in the direction parallel to the long side of the vacuum envelope  10 , and are arranged at regular intervals in the direction parallel to the short side. The shape of the support members  14  is not limited to this. They may be columnar members.  
      As shown in  FIGS. 2 and 3 , a phosphor screen  16  is formed on the inner surface of the front substrate  11 . The phosphor screen  16  includes rectangular phosphor layers R, G and B that are luminous in red, blue and green, respectively, and are arranged in a matrix, and a light shielding layer  20  provided between the phosphor layers. A metal back  17  of aluminum, for example, and getter film  19  are stacked in this order on the phosphor screen  16 .  
      A large number of electric-field emission type electron emission elements  22 , which emit respective electron beams, are provided on the inner surface of the rear substrate  12  as electron emission sources for activating the phosphor layers R, G and B. The electron emission elements  22  are arranged in rows and columns in units of pixels.  
      Since a high voltage is applied to the phosphor screen  16 , the plate glass as the material of the front and rear substrates  11  and  12  and support members  14  is high-distortion-point glass. As shown in  FIGS. 2 and 4 , the frame  13  is formed of a plurality of, e.g., two frame members  13   a  and  13   b . The frame members  13   a  and  13   b  are substantially L-shaped, and have their ends outwardly angled by about 45° with respect to the linear portions thereof. The frame members  13   a  and  13   b  are formed of a metal bar or wire having a circular section, and have their surfaces coated with a silver plate layer  15 . The frame member  13   a  has its ends abutting on the ends of the other frame member  13   b . The frame members  13   a  and  13   b  provide the rectangular frame  13 . The ends of the frame members  13   a  and  13   b  are overlapped each other in a direction parallel with the surface of the front substrate  11 , thereby forming joints  27 . The gap between the front substrate  11  and frame  13  and the gap between the rear substrate  12  and frame  13  are sealed with a sealing layer  33  formed by fusing a ground layer  31  provided on the sealing surfaces, and an indium layer  32  formed on and bonded to the ground layer. Further, the abutting ends of the frame members  13   a  and  13   b  are bonded by indium.  
      A method of manufacturing the FED constructed as the above will now be described in detail.  
      Firstly, the phosphor screen  16  is formed on plate glass serving as the front substrate  11 . More specifically, plate glass of the same size as the front substrate  11  is prepared, and a stripe pattern of a phosphor layer is formed on the plate glass using a plotter machine. The plate glass with the phosphor stripe pattern and plate glass for the front substrate are placed on a positioning jig and set on an exposure table, thereby performing exposure and development to produce the phosphor screen  16 .  
      Subsequently, the electron emission elements  22  are formed on plate glass for the rear substrate. Specifically, conductive cathode layers are formed in a matrix on the plate glass, and an insulation film of silicon dioxide is formed on the conductive cathode layers by, for example, thermal oxidation, CVD or sputtering. After that, a metal film for gate electrodes, which is formed of molybdenum or niobium, is formed on the insulation film by, for example, sputtering or electron-beam deposition. Thereafter, a resist of the pattern corresponding to the to-be-formed gate electrodes is formed by lithography. Using the resist pattern as a mask, the metal film is etched into gate electrodes  28  by wet or dry etching.  
      Subsequently, using the resist pattern and gate electrodes as masks, the insulation film is etched into cavities  25  by wet or dry etching. After removing the resist pattern, electron-beam deposition is performed on the surface of the rear substrate at a preset angle with respect to the surface, thereby forming, on the gate electrodes  28 , peel layers formed of, for example, aluminum or nickel. After that, a material, such as molybdenum, for cathodes is vertically deposited on the rear substrate surface by electron-beam deposition. As a result, an electron emission element  22  is formed in each cavity  25 . Subsequently, the peel layer and metal film thereon are removed by the liftoff method.  
      After forming the electron emission elements  22 , a plurality of support members  14  are provided on the rear substrate  12 . Thereafter, the frame members  13   a  and  13   b  providing the frame  13  are formed. The frame members  13   a  and  13   b  are each formed by bending a metal bar of a circular section into an L-shape in accordance with the size of the frame  13 . The frame members  13   a  and  13   b  are formed of, for example, an NiFe alloy having substantially the same thermal expansion coefficient as the glass panel of the front and rear substrates.  
      After that, the frame members  13   a  and  13   b  are plated with silver. Specifically, firstly, a round bar of an NiFe alloy is rinsed using pure water or alcohol, and dried. The round bar is then placed in a plating-liquid vessel and subjected to electrolytic plating, whereby the round bar is plated with a silver film of about 2 to 7 μm. Thereafter, the round bar is rinsed using pure water or alcohol, and dried. As a result, frame members  13   a  and  13   b  plated with a silver layer  15  are acquired.  
      The frame members  13   a  and  13   b  have the same size and shape and can be treated as common components. Further, since the frame  13  is formed of two members  13   a  and  13   b , the workability and moldability can be enhanced. The frame members plated with silver exhibit a high affinity to a sealing metal, described later, and can realize sealing of high airtightness.  
      Subsequently, the sealing surface of the front substrate  11 , which is formed of an inner peripheral edge portion thereof, and the sealing surface of the rear substrate  12 , which is formed of an inner peripheral edge portion thereof, are coated with silver paste by screen printing, thereby forming a frame-shaped ground layers  31 . The ground layer  31  is coated with indium as a conductive sealing metal layer, whereby an indium layer  32  is formed over the entire circumference of the ground layer. It is desirable to use, as the sealing metal, a low-melting-point metal having a low melting-point of about 350° C. or less and excellent in adhesion and bonding.  
      The first and second frame members  13   a  and  13   b  are placed on one of the substrates, e.g., on the indium layer  32  of the front substrate  11 . At this time, as shown in  FIG. 4 , the first and second frame members  13   a  and  13   b  are made to abut on each other to thereby form a rectangular frame. At the same time, the first and second frame members  13   a  and  13   b  are positioned with respect to the front substrate  11 .  
      After that, as shown in  FIG. 5 , the rear substrate  12  with the ground layer  31  and indium layer  32  provided on the sealing surfaces is opposed to the front surface  11  with the frame  13  on the indium layer  32 , with a preset distance defined therebetween. At this time, for example, the front substrate  1  is directed upward and placed below the rear substrate  12 . In this state, the front and rear substrates  11  and  12  are held by, for example, an assembly jig, and placed in a vacuum-processing apparatus.  
      As shown in  FIG. 6 , a vacuum-processing apparatus  100  comprises a load chamber  101 , baking/electron-beam-cleaning chamber  102 , cooling chamber  103 , getter-film deposition chamber  104 , assemblage chamber  105 , cooling chamber  106  and unload chamber  107 . Each chamber is constructed as a process chamber capable of vacuum processing, and all chambers are maintained in a vacuum state when producing an FED. Further, the adjacent process chambers are connected to each other by, for example, a gate valve.  
      After the front substrate  11  with the frame  13  and the rear substrate  12  are loaded into the load chamber  101 , the load chamber  101  is exhausted. Subsequently, the front and rear substrate  11  and  12  are sent to the baking/electron-beam-cleaning chamber  102 . In the baking/electron-beam-cleaning chamber  102 , when the degree of vacuum reaches approx. 10 −5  Pa, the rear and front substrates  12  and  11  are heated to approx. 300° C., thereby sufficiently discharging the gas absorbed in the surface of each substrate.  
      At this temperature, the indium layers (their melting-point is approx. 156° C.)  32  are melted. However, since the indium layers  32  are formed on the ground layers  31  that exhibit a high affinity thereto, indium does not flow and reliably adheres the first and second frame members  13   a  and  13   b  to the front substrate  11 . Indium spreads to the gaps between the first and second frame members  13   a  and  13   b . Note that since the sealing property may be degraded by the gas discharged from the frame members, it is desirable that the frame members be baked to discharge the gas before indium and the frame members are adhered. However, if the back process is performed at 200° C. or less, such pre-baking is not necessary. The front substrate  11  with the frame  13  adhered thereto will hereinafter be referred to as “the front-substrate-side assembly”.  
      In the baking/electron-beam-cleaning chamber  102 , during heating, an electron-beam generation unit, not shown, applies an electron beam to the phosphor screen of the front-substrate-side assembly and the electron-emission-element surface of the rear substrate  12 . Since the electron beam is deflected and scanned by a deflection unit mounted on the external portion of the electron-beam generation unit, it can clean the entire phosphor screen and electron-emission-element surface.  
      After electron-beam cleaning, the front-substrate-side assembly and rear substrate  12  are sent to the cooling chamber  103 , where they are cooled to approx. 100° C. Subsequently, the front-substrate-side assembly and rear substrate  12  are sent to the deposition chamber  104 , where a Ba film as a getter film is deposited on the outer surface of the phosphor screen. The Ba film prevents the surface from being contaminated by oxygen or carbon, thereby keeping it in an active state.  
      After that, the front-substrate-side assembly and rear substrate  12  are sent to the assemblage chamber  105 . In this chamber, current is flown to the indium layers  32  and frame members  13   a  and  13   b  to heat them, with the front-substrate-side assembly and rear substrate  12  stacked. As a result, the indium layers  32  are again melted into a liquid or softened, whereby the front-substrate-side assembly and rear substrate  12  are attached to each other in a sealed state via the frame  13  to provide the vacuum envelope  10 . Conduction heating is performed by, for example, providing electrodes at the joints  27  of the frame  13 .  
      The thus-formed vacuum envelope  10  is cooled down to the room temperature in the cooling chamber  106 , and is then unloaded from the unload chamber  107 . From the above-described process, the vacuum envelope of the FED is completed.  
      In the FED constructed as the above and its manufacturing method, the front and rear substrates  11  and  12  are attached to each other in a sealed state in a vacuum, and baking and electron-beam cleaning are performed to sufficiently discharge the gas absorbed in the surface of the substrates. Further, the getter film is also prevented from oxidation to thereby provide a sufficient gas absorption effect. As a result, an FED capable of maintaining a high degree of vacuum can be acquired.  
      The frame  13  comprises a plurality of, e.g., two, frame members. Compared to a rectangular frame formed of a single member, the frame members can be easily transferred, assembled or handled, and can be prevented from being deformed when they are transferred or assembled. At the same time, the workability or moldability of the frame can be enhanced. Accordingly, a frame of a desired shape can be attached in a sealed manner at a preset location with high accuracy, with the result that a reliable sealed state can be easily provided even for a large display unit of 50 inches or more, and hence high airtightness and mass productivity can be acquired. Further, plating the frame members with silver enhances the affinity of the frame members to the sealing member, thereby realizing highly airtight sealing.  
      The material of the frame members is not limited to an NiFe alloy. Other materials, which can be subjected to plating and have a thermal expansion coefficient relatively similar to that of the substrates, may be used. For instance, a conductive metal, such as Fe, Ni, Ti or stainless steel, or an alloy of such metals, or a non-conductive material, such as glass or ceramic, may be used. Furthermore, the cross section of the frame members is not limited to a circular one, but may be changed to a rectangular or elliptic one, when necessary. Yet further, the frame members are not limited to solid-core ones, but may be hollow. The plated material is not limited to silver, but may be gold, platinum, palladium or nickel, etc. It is sufficient if the plated material exhibits wettability with respect to the sealing material. If sufficient wettability of the frame members can be acquired, and vacuum sealing can be realized, the sealing material may be directly attached with no plated layer. The sealing material is not limited to indium, but may be an indium alloy or inorganic adhesive. Other materials may be used if they do not degrade the degree of vacuum or sealing.  
      The frame  13  is not limited to the two-piece one, but may be divided into three or more portions, namely, may be formed of three or more frame members. The frame members are not limited to L-shaped ones, but may have another shape, such as linear or U shape. The ends of the frame members may be obliquely angled or be straight.  
      For instance, in a first modification as shown in  FIG. 7 , the frame  13  comprises four frame members  13   a ,  13   b ,  13   c  and  13   d . The frame members  13   a ,  13   b ,  13   c  and  13   d  are formed linearly, and provide the long and short sides of the frame  13 . The opposite ends of each frame member are angled by substantially 45° with respect to the linear portion. At the corners of the frame  13 , the ends of the frame members  13   a ,  13   b ,  13   c  and  13   d  abut on each other in the direction parallel to the surface of the substrate. The joints  27  of the frame members are attached in a sealed state by a sealing material, and the gaps therebetween are filled with the material.  
      The joints  27  of the frame members may not always be provided at the corners of the frame  13 , but may be provided along the sides. In a second modification as shown in  FIG. 8 , the frame  13  comprises two frame members  13   a  and  13   b  that are substantially U-shaped. Along the long sides of the frame  13 , the ends of the frame members  13   a  and  13   b  abut on each other in the direction parallel to the surface of the substrate. The joints  27  of the frame  13  are attached in a sealed state by a sealing material, and the gaps therebetween are filled with the material.  
      In a third modification as shown in  FIG. 9 , the frame  13  comprises four frame members  13   a ,  13   b ,  13   c  and  13   d  that are substantially L-shaped. Along the short and long sides of the frame  13 , the ends of the frame members  13   a ,  13   b ,  13   c  and  13   d  abut on each other in the direction parallel to the surface of the substrate. The joints  27  of the frame  13  are attached in a sealed state by a sealing material, and the gaps therebetween are filled with the material.  
      As a fourth modification as shown in  FIG. 10 , the ends of the frame members  13   a  and  13   b  may be coupled to each other such that they overlap each other along a preset length in the direction perpendicular to the long sides of the frame and in the direction parallel to the surface of the substrate.  
      In the first to fourth modifications, the other elements of the FEDs are similar to those of the above-described embodiment. The similar elements are denoted by corresponding reference numbers, and are not described in detail. The first to fourth modifications can also provide the same advantage as the above-mentioned embodiment. In light of the positioning of the frame on the substrate, it is desirable to divide the frame into two portions or so. In contrast, in light of the easiness of transfer or handling of the frame, it is desirable to divide the frame into a large number of portions.  
      The present invention is not limited to the above-described embodiment and modifications, but may be further modified in various ways without departing from the scope. Various inventions can be realized by appropriately combining the structure elements disclosed in the embodiment and modifications. For instance, some of the disclosed structural elements may be deleted. Some structural elements included in the embodiment and modifications may be combined appropriately.  
      For instance, although in the embodiment, the electron emission elements are of a field emission type, they are not limited thereto, but may be other types of electron emission elements, such as pn-type cold cathode elements or electron emission elements of a surface conduction type. Further, the invention is also applicable to the manufacture of other image display apparatuses, such as plasma display panels or electroluminescence (EL) display panels.