FIELD EMISSION DISPLAY HAVING ADHESIVELY ATTACHED SPACERS AND ATTACHMENT PROCESS

Abstract: A field emission display (10) includes a plurality of spacers (24) attached to a cathode display plate (14) by an adhesive layer (38). The adhesive layer (38) is preferably an electrically conductive, transition-metal sol-gel adhesive. The sol-gel adhesive is formulated to provide an adhesive film (60) having predetermined surface properties, such that the adhesive film (60) can be spread upon a transfer stage (62). The adhesive film (60) possesses sufficient tackiness, such that vertical members (52) can be brought into contact with the adhesive film (60), whereupon a small amount of adhesive adheres to an attachment surface (58) at the end of the vertical members (52). After applying the adhesive, the vertical members (52) are directly attached to a bonding area (66) on display plate (54) by means of a moveable transfer device (40).

Description: FIELD EMISSION DISPLAY HAVING ADHESIVELY ATTACHED SPACERS AND ATTACHMENT PROCESS Field of the Invention The invention relates to flat panel display devices, and more particularly to field emission display devices having vertical structures positioned between two display plates . Background of the Invention A field emission display typically includes a cathode display plate, an anode display plate and an evacuated inner-space region between the two display plates. Electrons are emitted by emitter structures disposed on the cathode plate and are accelerated toward the anode display plate. A plurality of phosphors are disposed on the anode plate facing the evacuated inner- space region. Upon being struck by electrons from the cathode plate, the phosphors emit light through the anode plate. State of the art field emission displays are fabricated to have a diagonal dimension on the order of 5 - 7 inches. Additionally, to maintain a light overall product weight, and narrow thickness dimensions a typical sheet of glass having a thickness of about 0.04 inches is utilized for the display plates. At a large display screen area, the thin plates are not sufficient to withstand the pressure differential existing across the plates at standard atmospheric pressures. In a typical field emission display, the inner-space region is evacuated to a pressure on the order of 10 "6 torr. Consequently, spacers are inserted between the front plate and the back plate to counteract the differential pressure and avoid collapsing the display plates. The spacers prevent collapsing of the display plates and allow the display area to be increased with little or no increase in plate thickness. Although spacers perform an important and necessary function in field emission displays operating at high vacuum, the spacers must be precisely positioned within the field emission display to avoid degrading the display performance. For example, if a spacer is misaligned and overlaps an emitter structure, electrons emitted by the emitter structure will not reach the anode, but instead will be absorbed and deflected by the overlying spacer. In addition to the necessity for precise alignment, existing methods for attaching spacers during display assembly require complex bonding processes that are expensive and time consuming. Accordingly, a need exists for an improved field emission display having precisely aligned vertical structures, such as spacers, and the like, and a method for precise alignment of such vertical members to the display plates. Brief Description of the Drawings FIG. 1 illustrates, in cross-section, a field emission display arranged in accordance with the invention: FIG. 2 illustrates, in cross-section, an exemplary apparatus useful for carrying out a field emission display fabrication process in accordance with the invention; FIG. 3 illustrates, in cross-section, an exploded view of a spacer attachment process in accordance with the invention; FIG. 4 illustrates, in cross-section, a portion of a cathode display plate constructed in accordance with the invention. It will be appreciated that for simplicity and clarity of illustration, elements shown in the FIGURES have not necessarily been drawn to scale. For example, the dimensions of some of the elements are exaggerated relative to each other. Further, where considered appropriate, reference numerals have been repeated among the FIGURES to indicate corresponding elements. Detailed Description of Preferred Embodiments The present invention is for a field emission display having a plurality of vertical members disposed between two display plates. The vertical members are attached to one of the display plates by means of an adhesive material that secures each vertical member to the display plate. The adhesive material is preferable a sol-gel adhesive formulated to provide a tacky surface, such that the sol-gel will adhere to a bonding edge of the vertical members. The present invention further includes a process for fabricating a field emission display in which a layer of adhesive sufficient to bond the vertical members to a display plate is transferred to a bonding edge of the vertical members. Once the adhesive is applied to the vertical members, the vertical members are automatically positioned at predetermined locations on an interior surface of the display plate. A field emission display fabricated in accordance with the invention provides an improved alignment of vertical members, such as spacers, to a face plate, such as a cathode plate. By attaching spacers to a cathode plate using an adhesive material, alignment of the spacers between the cathode and the anode of a field emission display is improved. Improved spacer alignment provides a field emission display free of electron emitter obstruction that can occur when spacers are misaligned to the cathode. FIG. 1 illustrates, in cross-section, a field emission display 10 assembled in accordance with the invention. Field emission display 10 includes an anode display plate 12 and a cathode plate 14. Anode display plate 12 has an interior surface 16 generally situated in parallel spaced relationship with an interior surface 18 of cathode display plate 14. Side walls 20 are located at the perimeters of anode display plate 12 and cathode display plate 14 and extend there between to maintain a predetermined spacing between interior surfaces 16 and 18. An evacuated region 22 resides within the inner- space region created by facing interior surfaces 16 and 18, and by side walls 20. A plurality of spacers extend between anode display plate 12 and cathode display plate 14. Spacers 24 function to prevent collapsing of the display plates toward evacuated region 22 as a result of the differential pressure experienced by the display plates. Typically, the display plates and side walls are fabricated from a hard material, such as glass, ceramic, and the like. In one embodiment of the invention, evacuated region 22 has a vacuum pressure of about lxlO -6 torr or less. Anode display plate 12 includes a glass substrate 26 having a plurality of phosphors 28 deposited on interior surface 16. A layer of black surround 30 is also deposited on interior surface 16 in regions intermediate to phosphors 28. A reflective metal layer 32 overlies phosphors 28 and black surround 30 and faces evacuated region 22. Reflective metal layer 32 functions to improve the optical characteristics of field emission display 10, and also acts as an electrical conduit for the dissipation of excess charge built up on anode display plate 12 during operation of the field emission display 10. A plurality of electron emitters 34 are arrayed on interior surface 18 of cathode display plate 14. Electron emitters 34 are made from an electron- emissive material, such as molybdenum, niobium, tungsten, and carbon, and the like. Cathode display plate 14 and anode display plate 12 also include gate electrodes and appropriate electronics (not shown) , for selectively addressing electron emitters 34. Such gate electrode structures and electronic circuits are known to those skilled in the art. Spacers 24 are coupled to anode display plate 12 by bonding pads 36. Bonding pads 36 are disposed at locations on anode display plate 12 where it is desired to bond spacers 24. Bonding pads 36 can be formed by selectively depositing a metal, such as aluminum, using one of a number of standard metal film deposition techniques. The thickness of bonding pads 36 is preferably about 0.05 - 5.0 microns. Bonding pads 36 provide a compliant coupling between anode display plate 12 and spacers 24. The placement of compliant materials at the spacer contact area on anode display plate 12 prevents cracking or breakage by compensating for variation in the height of spacers 24. Additionally, in displays where the components are made from different materials, complaint materials at the contact region accommodate a variation in the coefficient of thermal expansion between spacers 24 and anode display plate 12. In accordance with the invention, spacers 24 are attached to interior surface 18 of cathode display plate 12 by an adhesive layer 38. Adhesive layer 38 is positioned on interior surface 18 at pre-designated spacer contact areas. The spacer contact areas reside intermediate to the groups of electron emitters 34 and extend in a generally parallel direction across cathode display plate 14. In a preferred embodiment of the invention, adhesive layer 38 is a sol-gel material formulated by hydrolyzing transition metal or organo metallic precursors, or both, to form an adhesive material. In formulating the sol-gel material, additives that alter the centering temperature, and the electrical conductivity of the adhesive can be introduced. For example, colloidial metal particles, such as silver, gold, and platinum, and the like, can be added to the sol-gel to achieve enhanced electrical conductivity. The colloidal metal particles can be obtained from halogenated metal precursors, such as gold chloride, and the like. Alternatively, the compounds containing colloidial metals can be obtained and directly introduced to the sol-gel solution. During formulation, the colloidal metal is precipitated within the sol-gel solution. A variety of metals can be introduced to the sol-gel, including transition metals, such as chrome, manganese, cobalt, and the like. Specific formulations of sol-gel adhesives will subsequently be described hereinafter. In an alternative embodiment, adhesive layer 38 can be a metal-based epoxy. For example, epoxy or resin based materials containing metal particles, such as silver, can be formulated to have the necessary physical properties to provide spacer attachment. In yet another alternative embodiment, a metal-glass paste can be used to form adhesive layer 38. An important advantage the adhesive attachment of spacers to display plates in a field emission display relates to the ability to readily automate the placement process. By formulating an adhesive material having a certain amount of tackiness and surface tension, vertical members can be brought into contact with adhesive material and directly attached to a display plate. Yet another advantage of the present invention relates to the low temperature processing conditions of the adhesive attachment process. Prior art bonding methods can require temperatures of 500 °C or more. These processing temperatures can damage the delicate metallurgy of the cathode plates typically used in a flat panel display. Shown in FIG. 2 is an exemplary embodiment of a spacer attachment mechanism suitable for practicing the process of the invention. A moveable transfer device 40 is configured to perform several process steps necessary for placement of spacers on a display plate. In the exemplary embodiment, a transfer assembly 42 is slideably mounted to a rotatable piston shaft 44. Transfer assembly 42 reciprocally travels along a rail assembly 46 mounted to piston shaft 44. A process table 48 contains gear mechanisms (not shown) for raising and lowering piston shaft 44. A heated vacuum chuck 50 is positioned at a terminal end of transfer assembly 42. Vacuum chuck 50 contains attachment mechanisms (not shown) for grasping and holding vertical member 52, while transferring the work piece between various staging areas. To attach a vertical member, such as a spacer, to a display plate 54, the display plate is positioned on a stage assembly 56. Electronic control systems (not shown) within process table 48 activate transfer assembly 42 to grasp a vertical member from a staging area (not shown) . Subsequently, transfer device 42 is activated to bring an attachment surface 58 of work piece 52 into contact with an adhesive film 60. Adhesive film 60 is evenly spread over a transfer stage 62 by movement of a doctor blade (not shown) across transfer stage 62. When attachment surface 58 is brought into contact with adhesive film 60, the action of surface and capillary forces cause a small amount of adhesive film 60 to transfer to attachment surface 58. It is important to note that the adhesive compound is formulated to have the necessary physical properties such that a small amount of adhesive will adhere to attachment surface 58 when vertical member 52 is brought into contact with the adhesive. Once adhesive is applied to attachment surface 58, transfer device 42 then rotates and translates to position vertical member 52 at a desired location on display plate 54. An optical system 64 interactively communicates with control electronics to precisely position vertical member 52 at a predetermined position on display plate 54. An exploded view of the attachment process performed in accordance with the invention is illustrated in FIG. 3. Vacuum chuck 50 is vertically maneuvered to bring attachment surface 58 of vertical member 52 into close proximity with a bonding area 66 located on the surface of display plate 54. Upon bringing attachment surface 58 into close proximity with bonding area 66, an adhesive contact 68 is formed by capillary forces acting to transfer adhesive film carried on vertical member 52 to bonding surface 66. It is important to note that it is unnecessary to apply strong vertical force to adhere vertical member 52 to bonding surface 66. A particular advantage of the present invention includes the use of an adhesive material that sufficiently wets bonding surface 66, such that capillary and surface forces act to transfer sufficient amounts of adhesive material bonding surface 66. Those skilled in the art will recognize that the foregoing attachment process can be used to attach a variety of vertical structures to a display plate. For example, charge stabilizing structures, charge deflecting structures, vertically oriented electron focusing structures, and the like, can be attached to a display plate by means of the inventive attachment process. Although, the embodiments illustrated herein describe the invention in the context of spacer alignment and attachment, the invention is in no way limited to such spacer structures. Shown in FIG. 4, in cross-section, is a portion of a cathode plate 70 having a plurality of spacers 72 positioned thereon. In accordance with the invention, spacers 72 are precisely positioned on cathode plate 70 relative to groups of emitter structures 74 overlying the surface of cathode plate 70. Adhesive bond 76 secure spacer 72 to cathode plate 70. Having been placed on cathode plate 70 by the inventive process illustrated in FIGs. 2 and 3, spacer 72 is precisely aligned to bonding areas adjacent to emitter structure 74. Subsequently, processing steps can be carried out to precisely align an anode plate to cathode plate 70. By first precisely placing spacers 72 on cathode plate 70, before attaching an anode plate, the overlap of spacers onto emitter structures 74 is avoided. As previously described, any misalignment associated with the attachment of an anode plate to cathode plate 70 will result in observable defects upon operation of the field emission display. In a preferred embodiment of the invention, adhesive film 60, used to form adhesive layer 38 and adhesive bonds 76, is prepared by means of a sol-gel process. Preferably, a transition metal sol-gel is prepared by combining a transition metal oxide precursor and a solvent, such as de-ionized water, ethanol, methanol, and the like. For example, oxides of transition metals, such as iron, titanium, tungsten, vanadium, and the like, can be used to prepare a conductive sol-gel. In a preferred embodiment, sodium vanadate (NaV0 3 ) is mixed with deonized water and an ion exchange resin to form a vanadate conductive sol-gel. A commercially available ion exchange resin can be used, such as "AMBERLITE JR 120 (plus)" available from ROHM and HAAS, Inc. of Philadelphia, PA. In formulating the sol-gel, the ion exchange resin replaces sodium with hydrogen in the vanadate compound. Preferably, about 0.1 - 2.0 grams of sodium vanadate are combined with about 10 to about 20.5 ml of deonized water and about 1 - 2 grams of ion exchange resin and mixed to produce the sol-gel. In a most preferred embodiment, 0.1 - 2 grams of sodium vanadate, 1 gram of ion exchange resin, and about 10 ml of water are mixed to form the sol-gel solution. After mixing, the solution is maintained at room temperature for a predetermined period to allow the hydrolization reaction to take place. Then, the solution is decanted to remove the sodium-filled ion exchange resin. After decanting, the sol-gel is ready for adhesive application and can be used in the field emission display fabrication process above. In an alternative embodiment the adhesive material is formulated from a silica sol-gel. The silica sol-gel is preferably prepared using a silicate compound, such as tetraethylorthosilicate (TEOS) , ethyl alcohol, and hydrochloric acid. Preferably, a sol-gel is prepared by adding about 30 ml of TEOS, 20 ml of ethyl alcohol, and about 1 ml of hydrochloric acid. After mixing, hydrolization reactions are induced by heating the mixture to about 105° C for about 2 hours. Upon completion of the hydrolization reaction the sol-gel material can be used in the fabrication of the field emission device in accordance with the previously described fabrication process. In either of the foregoing sol-gel preparation processes, metals, such as silver, gold, and platinum, and the like can be added to the sol-gel to produce an electrically conductive adhesive material. Alternatively, a precursor to the colloidial metal, such as gold chloride can be incorporated into the sol-gel. Subsequently, the colloidial metal can be precipitated within the sol-gel. Additionally, other transition metals, such as oxides of chrome, manganese, and cobalt, and the like, may be introduced to the sol-gel process in order to form an electrically conductive adhesive. Thus it is apparent that there has been provided, in accordance with the invention, a field emission display having adhesively attached spacers and an attachment process which fully meets the advantages set forth above. Although the invention has been described and illustrated with reference to specific illustrative embodiments thereof, it is not intended that the invention be limited to those illustrative embodiments. Those skilled in the art will recognize that variations and modifications can be made without departing from the spirit of the invention. For example, mechanisms substantially different from that illustrated herein can be used in the inventive attachment process. It is therefore intended to include within the invention all such variations and modifications as fall within the scope of the appended claims and equivalents thereof.

Claims: Claims We claim: 1. A field emission display (10) comprising: a display plate having an interior surface; a bonding area (66) on the interior surface; a vertical member (52) coupled to the bonding area (66) ; and an adhesive layer (38) intermediate to the bonding area (66) and the vertical member (52), wherein the adhesive layer (38) is selected from the group consisting of a sol-gel adhesive, a metal-glass paste, and a metal-filled epoxy. 2. The field emission display (10) of claim 1, wherein the sol-gel adhesive comprises hydrolyzed tetraethyl orthosilicate. 3. The field emission display (10) of claim 1, wherein the sol-gel adhesive comprises: hydrolyzed tetraethyl orthosilicate; and an alkali-metal oxide. 4. The field emission display (10) of claim 1, wherein the sol-gel adhesive comprises: hydrolyzed tetraethyl orthosilicate; and an alkali-metal carbonate. 5. The field emission display (10) of claim 1, wherein the display plate comprises a cathode plate (14). 6. A field emission display (10) comprising: a cathode plate (14) having an interior surface (18); an anode plate (12) having an interior surface (16) in parallel spaced relationship with the interior surface (16) of the cathode plate (14); a first contact area on the interior surface (18) of the cathode plate (14) and a second contact area on the interior surface (16) of the anode plate (12); a spacer structure (72) having first and second opposed edges disposed between the cathode plate (14) and anode plate (12), wherein the first opposed edge is coupled to the first contact area and the second opposed edge abuts the second contact area; and an adhesive layer (38) intermediate to the first contact area and the first opposed edge, wherein the adhesive layer (38) is selected from the group consisting of a sol-gel adhesive, a metal-glass paste, and a metal-filled epoxy. 7. The field emission display (10) of claim 6, wherein the spacer structure (72) comprises a bulk resistive material having a thermal expansion coefficient substantially similar to the thermal expansion coefficient of the cathode plate (14) and the anode plate (12) . 8. The field emission display (10) of claim 6 further comprising a metal layer overlying the second contact area intermediate to the second contact area and the second opposed edge. 9. A process for fabricating a field emission display (10) comprising the steps of: providing a display plate having a bonding area (66) thereon; providing a movable transfer device (40) having a vertical member (52) temporarily attached thereto, the vertical member (52) having an attachment surface (58); providing an adhesive film (60) selected from the group consisting of a sol-gel adhesive, a metal-glass paste, and a metal-filled epoxy; activating the movable transfer (40) device to bring the attachment surface (58) into contact with the adhesive film (60); activating the movable transfer (40) device to bring the attachment surface (58) into contact with the bonding area (66); transferring an amount of adhesive from the attachment surface (58) to the bonding area (66) ; and annealing the first plate to bond the attachment surface (58) to the bonding area ( 66) . 10. The process of claim 9, wherein the step of providing an adhesive film (60) comprises the steps of: hydrolyzing tetraethyl orthosilicate to form a sol-gel; and applying the sol-gel to a transfer stage.

Inventors: DEAN KENNETH A, KRIHAK MIKE, PENA SOLOMON R, SMITH PETER A, ZARBOCK EDWARD A, DEAN, KENNETH, A., KRIHAK, MIKE, PENA, SOLOMON, R., SMITH, PETER, A., ZARBOCK, EDWARD, A.
Applicants: MOTOROLA INC, MOTOROLA, INC.
Application year: 1999
Main classification: H01J29
IPC codes: H01J9, H01J29, H01J31