Abstract:
A method and apparatus for cleaning and sealing components of a display utilizes continuous isolation of the components between the cleaning step and the sealing step. This limits exposure of the components to contaminants and isolates the components from oxidizing agents which can cause an oxide to form on the surface of one or more of the components. In one embodiment, a high vacuum transfer station couples a cleaning station and a sealing station to allow a component to be transferred from the cleaning station to the sealing station without leaving the high vacuum. In another embodiment, the apparatus includes a conveyor transferring the components from the cleaning station at a high vacuum to the sealing station at a similarly high vacuum without exposure to the atmosphere. Within the cleaning station, the component is cleaned using any of a variety of conventional cleaning techniques, including anisotropic and isotropic etching techniques such as reactive ion etching, plasma etching or vapor hydrofluoric acid etching. A third embodiment employs a single chamber for cleaning and sealing.

Description:
STATEMENT OF GOVERNMENT INTEREST  
       [0001]     This invention was made with government support under Contract No. DABT-63-93-C-0025 awarded by Advanced Research Projects Agency (“ARPA”). The government has certain rights in this invention. 
     
    
     TECHNICAL FIELD  
       [0002]     The present invention relates to field emission displays, and more particularly to methods of packaging field emission displays.  
       BACKGROUND OF THE INVENTION  
       [0003]     Flat panel displays are widely used in a variety of applications, including computer displays. One suitable flat panel display is a field emission display. Field emission displays typically include a generally planar emitter substrate covered by a display screen. A surface of the emitter substrate facing the display screen has formed thereon an array of surface discontinuities projecting toward the display screen. In many cases, the surface discontinuities are conical projections, or “emitters” integral to the substrate. Typically, the emitters are grouped into emitter sets in which the bases of the emitters in each emitter set are commonly connected. Drive electronics may also be integrated into or onto the substrate to control the flow of current to the emitter sets.  
         [0004]     A conductive extraction grid is positioned above the emitters and driven with a voltage of about 30-120 V. The emitter drive electronics then selectively ground the emitter sets to provide a current path to ground. The voltage differential between the extraction grid and the grounded emitter sets produces an electric field extending from the extraction grid to the emitters having an intensity that is sufficient to cause the emitter sets to emit electrons.  
         [0005]     The display screen is mounted directly above the extraction grid. The display screen is formed from a glass panel coated with a transparent conductive material that forms an anode biased to about 1-2 kV. The anode attracts the emitted electrons, causing the electrons to pass through the extraction grid. A cathodoluminescent layer covers a surface of the anode facing the extraction grid so that the electrons strike the cathodoluminescent layer as they travel toward the 1-2 kV potential of the anode. The electrons strike the cathodoluminescent layer causing the cathodoluminescent layer to emit light at the impact site. Emitted light then passes through the anode and the glass panel where it is visible to a viewer.  
         [0006]     Operation and extended lifetime of the emitter substrate typically requires that the emitter substrate be isolated from contaminants, such as moisture or oxidizing agents. The emitter substrate is therefore placed within a package to protect and isolate the emitter substrate. The glass panel carrying the anode acts as a cover for the package and seals to the package to form an airtight body containing the emitter substrate.  
         [0007]     Prior to sealing, the emitter substrate is cleaned according to conventional cleaning techniques, such as plasma etching, reactive ion etching or vapor hydrofluoric acid etching. The cleaning process removes contaminants and removes oxidized layers from the emitter substrate. Once the emitter substrate is cleaned, it is removed from the cleaning station and transferred to a sealing station. At the sealing station, the glass substrate of the display screen is bonded to the package to form a sealed, airtight enclosure.  
         [0008]     Even though the emitter substrate is typically transferred from the cleaning station to the sealing station in a cleanroom, the emitter substrate and interior of the package are often subjected to contaminants, such as moisture and oxidizing agents. The contaminants can damage the emitter substrate during transfer or after the package has been sealed. Additionally, contaminants in the sealed package detrimentally affect the operation of the field emission display.  
         [0009]     Among the particularly problematic contaminants of field emission displays are oxidizing agents, such as oxygen. Oxidizing agents cause surface oxides to form on the emitter substrate and/or on the drive electronics. Such surface oxides can affect the emissive properties of the emitters and can impair operation of the drive electronics.  
       SUMMARY OF THE INVENTION  
       [0010]     A method and apparatus for cleaning and sealing a package containing an emitter substrate continuously maintains the emitter substrate and package in a contaminant-free environment from the completion of the cleaning through the sealing of the package. In one embodiment of the apparatus according to the invention, separate cleaning and sealing stations are coupled through a transfer station. Each of the cleaning, sealing, and transfer stations is in a high vacuum chamber having a vacuum port to allow pumping of the chamber. The embodiment also includes a load lock chamber linked to the transfer station to allow transfer of packages into the transfer station.  
         [0011]     In a method according to this embodiment, housings containing emitter arrays are placed in the first load lock chamber. Then, the first load lock chamber is pumped to a high vacuum level. When the load lock chamber reaches the high vacuum level, the housings pass through a high vacuum link to the transfer station. The transfer station is then pumped to remove any contaminants, such as oxidizing agents.  
         [0012]     The housings then pass through a second link to the cleaning station where they are cleaned in a high vacuum. During, and at the completion of cleaning, the cleaning station is pumped to remove any additional contaminants, such as cleaning byproducts.  
         [0013]     After the housings and emitter arrays are cleaned, they pass through the second link to the transfer station and then through a third link to the sealing station. Within the sealing station, covers are placed atop the housings and sealed to form sealed packages containing the emitter arrays. Because the cleaning, transfer, and sealing stations are maintained at high vacuum, the arrays are maintained in a contaminant-free environment from the completion of cleaning through the sealing of the packages. Once the packages are sealed, they return through the third link to the transfer station. The sealed packages then move to the load lock chamber. The load lock chamber is then raised to atmospheric pressure and the sealed packages are removed.  
         [0014]     In a second embodiment of the apparatus according to the invention, a conveyor system transports packages through a cleaning station and a sealing station that is directly linked to the cleaning station through a high vacuum link. A first load lock chamber provides access for packages to enter the cleaning station and a second load lock chamber allows access for packages to exit the sealing station.  
         [0015]     In a method according to this embodiment, housings containing emitter substrates enter the first load lock chamber. Then the first load lock chamber is reduced to a high vacuum level and the housings are transferred to the cleaning station. When the emitter substrates are in the cleaning station, a cleaning gas or vapor is introduced to clean the housings and emitter substrates. Before completion of the cleaning step, the cleaning station is pumped to a high vacuum and substantially all contaminants are removed from the cleaning station. Then, the housings and emitter substrates are transferred to the sealing station where covers are attached and sealed to form sealed packages. The sealed packages then exit the sealing station to the second load lock chamber. Finally, the second load lock chamber is raised to atmospheric pressure, and the sealed packages are removed.  
         [0016]     A third embodiment of the apparatus according to the invention includes a single station that operates as both a cleaning and sealing station. Load lock chambers coupled to the cleaning and sealing stations allow insertion of housings and covers and removal of sealed packages. In a method according to this embodiment, housings and emitter substrates enter the first load lock chamber, and the first load lock chamber is pumped to a high vacuum level. Covers enter the second load lock chamber, and the second load lock chamber is pumped to approximately the same high vacuum level. The covers, housings and emitter substrates then enter the cleaning and sealing station, which is also at the high vacuum level. Within the cleaning and sealing station the emitter substrate is first cleaned. Before completing the cleaning process, the cleaning and sealing station is pumped again to remove contaminants, such as oxidizing agents and cleaning byproducts. While the covers, housings and emitter substrates are within the cleaning and sealing station, the covers are attached to the housings and sealed to form sealed packages. The sealed packages are removed through the third load lock chamber. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]      FIG. 1  is a block diagram of a first embodiment of a cleaning and sealing system according to the invention including a transfer station.  
         [0018]      FIG. 2  is a flowchart presenting processing steps in cleaning and sealing a package according to the invention.  
         [0019]      FIG. 3  is a block diagram of a cleaning and sealing apparatus according to the invention, including a conveyer.  
         [0020]      FIG. 4A  is a side cross-sectional view of an emitter array mounted in the housing of a display package.  
         [0021]      FIG. 4B  is a side cross-sectional view of a display screen bonded to a display housing to form a sealed display package.  
         [0022]      FIG. 5  is a block diagram of a third embodiment of a cleaning and sealing system according to the invention including a combined cleaning and sealing station. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0023]     As shown in  FIG. 1 , a package sealing system  40  includes a cleaning station  42  and a sealing station  44  linked by a transfer station  46 . The cleaning station  42  is a conventional integrated circuit cleaning structure, such as a plasma-etching chamber, reactive-ion etching chamber or by vapor hydrofluoric acid etching. To allow cleaning at extremely low pressure, the cleaning station  42  is vacuum sealable, and includes a vacuum port  48  at which a high vacuum, typically about 0.01-300 mTorr, can be applied through conventional vacuum pumping. As will be discussed below, the cleaning station  42  is typically maintained at a high vacuum during normal operation.  
         [0024]     The sealing station  44  is of a conventional type allowing packages to be sealed in a vacuum. To allow sealing at a high vacuum, the sealing station  44  includes a vacuum port  50  to which a high vacuum can be applied through conventional vacuum pumping. Like the cleaning station, the sealing station is maintained at high vacuum during normal operation.  
         [0025]     The cleaning station  42  and sealing station  44  are linked to each other by a transfer station  46 . The transfer station  46  is a high vacuum sealable chamber linked to each of the cleaning station  42  and sealing station  44  by respective high vacuum links  52 ,  54 . Like the cleaning and sealing stations  42 ,  44 , the transfer station  46  also includes a vacuum port  56  to allow the transfer station to be pumped to a high vacuum.  
         [0026]     The links  52 ,  54  are conventional links between high vacuum chambers, such as resealable passageways. One skilled in the art will recognize a variety of structures and methods for transferring parts between the transfer station  46  and the cleaning and sealing stations  42 ,  44  while maintaining a vacuum. For example, the transfer station  46  may include “turntable” structures or conveyer systems linking the stations  42 ,  44 . The turntables or conveyor systems transport the parts along the vacuum sealed passageways forming the links  52 ,  54 . Typically, the links  52 ,  54  include resealable doors to isolate the stations  42 ,  44 ,  46  before and after transfer of parts.  
         [0027]     In addition to the stations  42 ,  44 ,  46 , the sealing system  40  also includes a load lock chamber  58  linking the transfer station  46  to the external atmosphere. The load lock chamber  58  is a conventional load lock chamber linked to the transfer station  46  through a high vacuum link  60 . The load lock chamber  58  also has an insertion port  62  for inserting parts. The load lock chamber  58 , like the stations  42 ,  44 ,  46 , further includes a vacuum port  64  to allow the load lock chamber  58  to be pumped to a high vacuum.  
         [0028]     Operation of the sealing system  40  of  FIG. 1  is best explained with reference to the flowchart of  FIG. 2  and the cross sectional representations of a display  68  in  FIGS. 4A and 4B . Prior to reaching the sealing system  40 , an emitter substrate  70  is mounted in a recess  72  in a display housing  74 , as represented in step  200  of  FIG. 2  and shown in  FIG. 4A . The housing  74  containing the emitter substrate  70  is then transferred to the load lock chamber  58  ( FIG. 1 ) which is pumped down to a high vacuum. The transfer station  46  is also at the high vacuum at this point.  
         [0029]     Once the load lock chamber  58  reaches the high vacuum and the pressures in the load lock chamber  58  and transfer station  46  are about equal, the housing  74  and substrate  70  are transferred through the link  60  to the transfer station  46  in step  204 . As noted above, the cleaning station  42  is also at a high vacuum. Once the transfer station  46  reaches the high vacuum and the pressures in the transfer station  46  and cleaning station  42  are about equal, the housing  74  and substrate  70  are transferred to the cleaning station  42  through the link  52  in step  208 . Within the cleaning station  42 , the substrate  70  and housing  74  are cleaned according to conventional techniques, such as plasma etching, reactive ion etching or vapor hydrofluoric acid etching. During, and at the completion of, the cleaning process, the cleaning station  42  is pumped down through the vacuum port  48  to evacuate contaminants, such as cleaning byproducts, residue, oxides, and/or cleaning agents, in step  212 .  
         [0030]     At the completion of cleaning, the housing  74  and substrate are transferred through the link  52  from the cleaning station  42  to the transfer station  46  in step  214 . Then, the housing  74  and substrate  70  are transferred through the link  54  from the transfer station  46  to the sealing station  44  in step  218 . Because the cleaning station  42 , transfer station  46 , and sealing station  44  are all at high vacuum, these transfers occur with substantially complete isolation from the outside atmosphere. Consequently, the emitter substrate  70  is not exposed to oxidizing agents, such as contaminants or oxygen in the air. The substrate  70  thus does not develop surface oxides that can impair its performance. Moreover, because the system incorporates the load lock chamber  58 , the stations  42 ,  44 ,  46  are not vented to the outside environment, further reducing risk of exposure to contaminants.  
         [0031]     Within the sealing station  44 , a transparent cover  76  is placed atop the housing  74  in step  220 . As shown in  FIG. 4B , the cover  76  is formed from a glass plate  78  having a transparent anode  80  and cathodoluminescent layer  82  on an inner surface. In step  222 , the cover  76  is bonded to the housing  74  with a bonding agent  84  that may be a glass solder or frit, or other conventional bonding agent. The sealed cover  76  and housing  74  thus form a sealed package  86 . Because sealing occurs within the evacuated sealing station  44 , the interior of the sealed package  86  is also evacuated. Consequently, the array  70  remains continuously isolated from contaminants between the cleaning step  212  and the sealing step  224 .  
         [0032]     Once the package  86  is sealed, the package  86  passes through the link  54  to the transfer station  46  in step  224 , and then through the link  60  to the load lock chamber  58 . The pressure in the load lock chamber  58  is then increased to atmospheric pressure in step  228 , and the package  86  is removed from the load lock  58  through the insertion port  62  in step  230 .  
         [0033]      FIG. 3  shows another embodiment of the package sealing station  40  according to the invention in which packages pass linearly through the sealing station  40  in a conveyor-like approach. The sealing system  40  includes an input lock chamber  90 , the cleaning station  42 , the sealing station  44 , and an output load lock chamber  102  all sequentially coupled by respective links  96 ,  98 ,  100 . Each of the load lock chambers  90 ,  102  includes a respective variable vacuum port  94 ,  104  and each of the stations  42 ,  44  includes a respective high vacuum port  48 ,  50 .  
         [0034]     In the embodiment of  FIG. 3 , the housings  74  ( FIG. 4A ) containing the emitter substrates  70  enter the input load lock chamber  90  through an insertion port  92 . The input load lock chamber  90  is then pumped to the high vacuum level through the vacuum port  94 . When the first load lock chamber  90  reaches the high vacuum level, the housing  74  and substrate  70  are transferred on a conveyor system  93  through the high vacuum link  96  to the cleaning station  42 . The substrate  70  is then cleaned, as described above.  
         [0035]     Once the substrate  70  is cleaned, the housing  74  and substrate  70  are conveyed through a high vacuum link  98  into the sealing station  44 . The sealing station  44  has previously been pumped to a high vacuum through the vacuum port  50  so that the housing  74  and substrate  70  undergo little or no pressure change when passing through the link  98 . Within the sealing station  44 , the cover  76  ( FIG. 4B ) is placed over the housing  70 . The package  86  is -then sealed as described above.  
         [0036]     Once the package  86  is sealed, the package  86  is conveyed through a vacuum link  100  to a second load lock chamber  102 . The pressure in the output load lock chamber  102  is then reduced to atmospheric pressure through a vacuum port  104 . Once the load lock chamber  102  reaches atmospheric pressure, the package  86  is removed through an extraction port  106 . This system  40  advantageously eliminates the high vacuum transfer station  46  of the embodiment of  FIG. 1 .  
         [0037]     In a third embodiment of the invention, shown in  FIG. 5 , the packages  86  are both cleaned and sealed at the cleaning station  42 . This system  40  includes the cleaning station  42  as the central unit. Three load lock chambers  112 ,  114 ,  116  provide access to the cleaning station  42 , and a vacuum port  118  allows the cleaning station  42  to be pumped to a high vacuum level.  
         [0038]     In operation, the first load lock chamber  112  is initially open to the atmosphere. Housings  74  and substrates  70  ( FIG. 4A ) are placed in the first load lock chamber  112 , and the first load lock chamber  112  is pumped to a high vacuum. At about the same time, covers  76  are placed in the second load lock chamber  114 . The second load lock chamber  114  is then pumped to a high vacuum.  
         [0039]     Once the first and second load lock chambers  112 ,  114  reach the high vacuum, the housings  74  and substrates  70  are transferred through a first link  120  to the cleaning station  42 . Covers  76  are transferred into the cleaning station  42  through a second link  122 . In the cleaning station  42 , the substrates  70  are cleaned as described above. During, and at the completion of cleaning, the cleaning station  42  is pumped down through the vacuum port  118  to a high vacuum to evacuate contaminants.  
         [0040]     When cleaning is complete, the covers  76  are placed on the housings  74 , and the packages  86  are sealed as described above. Then, the sealed packages  86  are transferred to the third load lock chamber  116 , which is also at a high vacuum. The third load lock chamber  116  is then raised to atmospheric pressure, and the packages  86  are removed.  
         [0041]     From the foregoing, it will be appreciated that, although embodiments of the invention have been described herein for purposes of illustration, various modifications may be made without deviating from the spirit and scope of the invention. For example, in the embodiment of  FIG. 1 , housings  74  may be transferred from the cleaning station  42  directly to the sealing station  44  through a direct path  110  (represented by the broken line in  FIG. 1 ) which may be an additional high vacuum link. Also, the cleaning step  212  has been described as reactive ion etching, plasma etching or vapor hydrofluoric acid etching. However, various other contaminant removal steps may be within the scope of the term cleaning. For example, steps such as rinsing with a cleansing or etching solution, ion milling, or various forms of isotropic or anisotropic etching would be within the scope of the term cleaning. Also, although the packages  86  have been described as being sealed in a high vacuum, one skilled in the art will understand that the packages  86  can be sealed in a different controlled environment. For example, selected contaminant-free gases, such as noble gases or nitrogen can be added to the sealing station  44  to equalize pressure across the cover  76  for some applications. Accordingly, the invention is not limited except as by the appended claims.