Abstract:
A vacuum envelope that can improve the vacuum degree in a field emission device is provided. The vacuum envelope includes the cathode side substrate  2  on which field emission elements are formed and the anode substrate  1  spaced by a predetermined distance in the electron emission direction. At least two openings are formed before sealing the vacuum envelope. The remaining gas is ousted from the vacuum envelope by introducing a high temperature gas inside the vacuum envelope for a predetermined period of time. Thereafter, one of the openings is sealed while the envelope is being evacuated to a vacuum state through the remaining openings.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a vacuum envelope that houses electron sources and electrodes each for gathering electrons emitted from an electron source. Particularly, the present invention relates to a flat vacuum envelope that houses field emission elements (field emission cathodes) each acting as an electron source and to a method for evacuating the same. 
     2. Description of the Related Art 
     Recently, the field emission electronic equipment, which includes a large number of micro field emission elements contained in a glass vacuum envelope and integrated in a vacuum micro-structure, is proceeding toward practical use as a vacuum microelectronic element. 
     As applications of the vacuum microelectronics technology, field emission devices including flat field emission display panels, pick-up tubes, electron beam lithography apparatuses, and the equivalents have been studied. 
     In a flat display panel embodying field emission elements, one pixel corresponds to a specific number of micro-cold cathodes (emitters). 
     Various types of cathodes including field emission elements, MIN-type electron emission elements, surface conduction-type emission elements, PN-junction-type electron emission elements, and others, each having a pointed end, have been proposed as the micro cold cathode. 
     As one most typical example, a field emission device (FED) is disclosed in “NIKKEI ELECTRONICS”, No. 654, Jan. 29, 1996, pp. 89-98. In the FED device, the so-called Spindt type field emission element (FED) is well known. 
     In the Spindt field emission element, a large number of emitter electrodes E are formed on the cathode substrate K, as shown in FIG.  6 . An insulating layer SiO 2  is laid over the cathode substrate K. A gate electrode GT is vapor-deposited over the insulating layer. Holes are formed in the gate electrode so as to expose the point of an emitter electrode E via each hole. 
     When a voltage Vgk is applied between the cathode electrode K and the gate electrode GT, the point of emitter electrode E emits electrons. An anode electrode A is placed so as to confront the cathode electrode K in the vacuum space. When an anode voltage Va is applied between the cathode electrode K and the anode electrode A, the anode electrode (A) gathers the emitted electrons. The field emission elements are arranged in group. When stripe gate electrodes are sequentially scanned while image signals are supplied to stripe cathode electrodes, the fluorescent materials coated on the cathode electrodes glow so that the display device operates as an indicator. 
     FIG.  7 ( a ) is a perspective view illustrating the envelope of the above mentioned display panel. FIG.  7 ( b ) is a side cross sectional view illustrating the envelope of the above mentioned display panel. 
     Referring to FIGS.  7 ( a ) and  7 ( b ), reference numeral  1  represents a glass substrate on the side of the anode (hereinafter referred to as an anode substrate) and  2  represents a glass substrate on the side of the cathode (hereinafter referred to as a cathode substrate). Micro-field emission elements are formed on the anode substrate so as to confront the cathode substrate. Anode electrodes are formed on the anode substrate so as to confront the cathode substrate. 
     The getter substrate  3  has the lower surface on which an exhaust hole  3   a  is formed to evacuate the inside of the envelope to a vacuum state. The getter  4  is for example, an evaporation-type getter. The getter is flashed at a high temperature after evacuating the envelope so that the inside of the envelope can be maintained to a high vacuum degree. 
     The juxtaposed structure of the cathode substrate  2  and the anode substrate  1  are sealed with a fritted glass  5  while the cathode substrate  2  is spaced from the anode substrate  1  by a small distance of 200 μm to 500 μm apart. The substrates  1  and  2  are generally arranged to be mutually shifted. Thus, the cathode electrode leads and gate electrode leads of the field emission elements can be placed to the portions where the substrates  1  and  2  do not confront from each other. 
     In the case of color displaying, anode electrode leads can be arranged on the cut portion (not shown) protruding toward the anode substrate. 
     As described above, the gap between the fringe of the cathode substrate  2  and the fringe of the anode substrate  1  are sealed with a fritted glass  5 , except the getter substrate  3 . An exhaust tube (not shown) is connected to the getter substrate  3  to evacuate the inside of the envelope by a vacuum pump. 
     In the vacuum envelope contains field emission elements, the cathode substrate  2  is separated from the anode substrate  1  by a small distance. In order to maintain the space of the envelope in a high vacuum degree, the evaporation-type getter  4  is generally disposed in the getter room. The getter  4  is vaporized by externally heating it at a high temperature. A getter mirror, which can adsorb the residual gas ousted from the electronic material or adsorbed after the evacuation step, is formed over the entire surface of the getter room. 
     In the flat display panel, since the very narrow space (t) of the vacuum envelope has a poor conductance to the gas flowing it, it is difficult that a vacuum pump draws the vacuum space to a high vacuum degree. 
     The ratio of the material for forming the existing field emission elements to the volume of the vacuum space is high. Hence, the evacuating process must be performed for a long time to bring the envelope to a predetermined vacuum degree by exhausting the remaining gas (particularly, moisture) adsorbed inside of the constituent materials. 
     In order to achieve a higher vacuum degree, the well-known getter flashing is performed after the evacuation process. Thereafter, the whole vacuum envelope is placed in an oven at about 200° C. for several hours to adsorb the remaining gas in the vacuum envelope. This makes the fabricating process more complex. The long evacuating step (e.g. 220 minutes) prolongs the product completion time. 
     SUMMARY OF THE INVENTION 
     The present invention is made to solve the above-mentioned problems. 
     Moreover, the objective of the invention is to provide a vacuum envelope that can improve the vacuum degree in a field emission device. 
     Another objective of the present invention is to provide a vacuum envelope evacuating method that can effectively evacuate gas remaining in the vacuum envelope. 
     The objective of the present invention is achieved by a vacuum envelope comprising a first substrate formed of a glass substrate; a second substrate arranged so as to confront the first substrate; and a side wall for separating the first substrate from the second substrate by a predetermined distance to form a space therebetween; wherein a first opening used to evacuate the inside of the envelope is formed in a part of a vacuum envelope assembled by the first substrate, the second substrate and the side wall; and wherein a second opening is formed in a part of the vacuum envelope, the second opening being sealed at a different position of the vacuum envelope, different from the position of the first opening. Thus, before sealing in a vacuum state, the envelope is backed while high temperature gas is being flowed using the first opening and the second opening. 
     According to the present invention, the vacuum envelope further comprises field emission elements formed on the first substrate and an anode electrode formed on the second substrate so as to confront the field emission elements. 
     According to the present invention, the vacuum envelope further comprises a getter room placed so as to cover the first opening. 
     Furthermore, a method for evacuating a vacuum envelope, comprises the steps of juxtaposing a first substrate and a second substrate so as to be spaced from each other a predetermined distance apart, the first substrate on which field emission elements are formed; temporarily framing the periphery of the first substrate and the periphery of the second substrate with fritted glass to form an envelope; introducing a gas at a high temperature for a predetermined period of time to flow through the envelope; sealing an outlet, except a main opening into which the gas is introduced; and evacuating the inside of the envelope to a vacuum state through the main opening, so that the envelope is maintained in a vacuum state. 
     According to the present invention, the method further comprises the step of previously forming at least two openings on a side portion of the envelope temporarily assembled. 
     In the method according to the present invention, the gas at a high temperature is selected from the group consisting of CO (carbon monoxide), N 2 , H 2 , and a mixed gas of an inert gas and CO, N 2 , or H 2 . 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     This and other objects, features, and advantages of the present invention will become more apparent upon a reading of the following detailed description and drawings, in which: 
     FIG. 1 is a schematic diagram illustrating an envelope evacuating method according to the present invention; 
     FIGS.  2 ( a ),  2 ( b ) and  2 ( c ) are diagrams for explaining the process of evacuating a vacuum envelope; 
     FIG. 3 is a perspective view illustrating a flat vacuum envelope; 
     FIGS.  4 ( a ),  4 ( b ) and  4 ( c ) are diagrams for explaining the process of evacuating a flat envelope; 
     FIGS.  5 ( a ) and  5 ( b ) are diagrams explaining high temperature gas flowing inside an envelope; 
     FIG. 6 is a schematic perspective view partially illustrating a vacuum envelope; and 
     FIGS.  7 ( a ) and  7 ( b ) are diagrams explaining a field emission element. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     An embodiment of the present invention will be described below with reference to the attached drawings. 
     FIG. 1 shows a device embodying the envelope evacuating method according to the present invention. 
     Referring to FIG. 1, reference numeral  10  represents a vacuum envelope of which the inside space is not in a sealed state, or in a pre-completion state. In FIG. 1, the same constituent elements as those of FIG. 7 are represented with like numerals. 
     In a sealing chamber  11 , the vacuum envelope  10  is fixed with a supporting tool (not shown) and is heated by a heating apparatus. The sealing chamber  11  can be constructed of a furnace that can heat the sealing chamber  11  at temperatures at which the fritted glass  5  is melted. 
     An intake and exhaust chamber  12  is equipped under the sealing chamber to blow a high temperature gas into the vacuum envelope  10  or to evacuate the inside of the vacuum envelope  10  (as described later). 
     An elevating rod  13  ascends and descends when a pressure is applied to the cylinder room  13   b . One end of the elevating rod  13  is formed of a head  13   a  on which a sealing body  17  for sealing the vacuum envelope  10  is placed. 
     A vacuum pump  14  is controlled to evacuate the inside of the intake and exhaust chamber  12  through the second valve  15 . 
     A first valve  16  is opened to introduce a gas at a high temperature into the intake and exhaust chamber  12  (in the arrow direction). 
     The end of the elevating rod  13  is supported in the inner cylinder  18  and sidably driven by a drive mechanism (not shown). A flexible sealing body  18   a  is placed on the end of the cylinder  18  to hermetically seal with the getter room  3  when it is contacted against the getter room of the vacuum envelope  10 . The cylinder chamber  13   b  vertically moves the elevating rod  13 . 
     According to the present invention, when the vacuum envelope  10  is conveyed into the sealing chamber  11 , the opening is left around the periphery of the fritted glass  5  laminated on the vacuum envelope  10 . Hence, as described later with reference to FIG. 2, a high vacuum space can be obtained while the gas remaining inside the vacuum envelope  10  is being evacuated. 
     That is, as shown in FIG.  2 ( a ), the inner cylinder  18  is first lifted to be in strong contact with the vacuum envelope  10  conveyed inside the sealing chamber  11 . In such a state, the first valve  16  is opened to introduce gas at a high temperature into the vacuum envelope  10 , as shown by the arrows. 
     Since the fritted glass  5  to be formed as the side wall of the vacuum envelope  10  is not completely sealed, the gas charged into the envelope  10  flows through the space between the first substrate  11  and the second substrate  12  in the arrow direction. The gas flows through the space in the vacuum envelope  10  and then is discharged out through the fritted glass portion  5  not sealed. 
     The flow of the high temperature gas allows the gas contents (mainly, moisture) remaining inside the envelope  10  to be exhausted sufficiently. 
     The gas temperature depends on the volume of the envelope  10  and is preferably 300° C. to 500° C. The gas flowing time depends on the temperature and is preferably several minutes to several hours. 
     After the high temperature gas is sufficiently flowing, the sealing chamber  11  is controlled to an elevated temperature. Thus, the fritted glass  5  applied to the peripheral portion of the envelope  10  is melted. Then the gas flowing through the envelope  10  is stopped. 
     At this time, it can be detected that the peripheral portion of the envelope  10  has been completely sealed with the fritted glass  5  by monitoring the pressure of the high temperature gas supplied. 
     After the complete sealing state is ascertained, the first valve  1  is closed to stop supplying the high temperature gas while the second valve  15  is opened. 
     The second valve  15  forms an exhaust passage to the vacuum pump  14 . The vacuum pump  14  evacuates the gas remaining inside the envelope  10  in the arrow direction. The envelope  10  is evacuated, for example, to a vacuum degree of 10 −3  to 10 −5  Pa. 
     After the envelope  10  is evacuated to a sufficient vacuum state, the elevating loader  13  is lifted as shown in FIG.  2 ( c ). Thus, the glass sealing body  17  placed on the head  13   a  is pushed against the exhaust inlet  3   a  of the getter room of the envelope  10 . The heating device inside the sealing chamber  11  welds the portion around the exhaust inlet  3   a  with the sealing body  17 . 
     In this welding step, the envelope  10  is maintained in a vacuum state. The envelope  10  is fed out by means of a conveying mechanism (not shown). Thereafter, the next envelope is conveyed into the evacuating chamber. 
     According to the same fabrication process, a flat envelope for a display panel can be fabricated. In that embodiment, since the exhaust inlet  3   a  of the vacuum envelope is formed with the getter room, the envelope can be increased to a higher vacuum degree by flashing the evaporation-type getter, in a similar manner to that to the common vacuum envelope. Thus, the envelope can be sustained to a higher vacuum state. 
     FIG. 3 is a perspective view illustrating the envelope  20  according to another embodiment of the present invention. 
     In this embodiment, the envelope  20  is formed of a first substrate  21  having the inner surface on which field emission elements are formed, a second substrate  22  is arranged so as to confront the field emission elements and having anode electrodes for gathering electrons emitted from the field emission elements are formed, and a side wall  23  for hermetically sealing the space between the first substrate  21  and the second substrate  22 . 
     As shown in FIG. 3, the envelope  20  has a first opening  24   a  and a second opening  24   b  opened vertically in the sidewall  23 . Before the sealing step, the sucked gas flows from the first opening  24   a  to the second opening  24   b.    
     Clips (or tapes)  25 ,  25 , . . . temporarily fix the first glass substrate  21  and the second glass substrate  22 . Glass sealing body  26   a  is a member used for sealing the first opening while the glass sealing body  26   b  is a member used sealing the second opening. 
     The glass sealing members ( 26   a,    26   b  ) are respectively supported by the welding heating members ( 27   a,    27   b  ) and are welded to hermetically seal the inside of the envelope after the evacuating step (as described later). 
     In the embodiment, a vacuum envelope  20  is completed by flowing a gas, e.g. CO, N 2 , H 2 , or a mixture of one of them and an inert gas, at a high temperature, into the inside of the envelope and then evacuating the envelope to a high vacuum state. 
     That is, as shown in FIG.  4 ( a ), first, the heating members  27   a  and  27   b  are separately disposed at both the ends of the envelope  20 . A high temperature gas is charged into the envelope  20  via the inner cylinder  20 , in the arrow direction. The gas passing through the envelope  20  is discharged from the opening  24   a.    
     By flowing the high temperature gas, the degassing and evacuating preliminary step is performed which blows out gas contents adhered on and left inside the envelope and sweeps out moisture adhered on various devices or material contained in the envelope. 
     Next, as shown in FIG.  4 ( b ), the heating member  27   a  on the side of the second opening  24   a  butts against the sealing member  26   a.  The second opening  24   a  is welded and sealed with the sealing member  26   a  through the heating operation. 
     After the completion of the welding step, the vacuum pump is driven to evacuate the inside of the envelope  20  from the first opening  24   b.  When the inside of the envelope is evacuated to a sufficient vacuum state, the first opening  24   b  is sealed with the sealing member  26   b.  Thereafter, the vacuum envelope is detached from the inner cylinder  18  and then is taken out of the sealing chamber. 
     That embodiment evacuates the getter room  3  but requires the flat inner cylinder  18  which directly blows and exhausts a gas at a high temperature from the opening formed in the side wall of the envelope. However, a very flat, slim vacuum envelope can be fabricated. 
     After the evacuation step, tape-like non-evaporation-type getters or flat, wire evaporation getters may be previously incorporated at the four corners of the envelope. Thus, the unwanted residual gases can be adsorbed by activating the getter after formation of the envelope. 
     As described above, the present invention is characterized in that high temperature gases are flown through the inside of the envelope in the previous evacuating step. In the envelope, at least two openings must be previously formed to improve the residual gas sweeping effect due to the high temperature gas flowing operation. 
     In order to flow gas smoothly in the flat space, it is required to effectively match the gas pressure, the opening area, and the viscosity resistance of the flowing path. 
     As well known in the vacuum technology, the flow of gas becomes turbulent at a high gas pressure, becomes a viscosity flow at a low gas pressure, and becomes a molecular flow at a lower gas pressure. 
     According to the present invention, it is preferable to increase the residual gas exhausting effect by decreasing the conductance to gas flowing in the envelope, as shown in FIGS.  5 ( a ) and  5 ( b ) and by setting the gas pressure, the positions of openings H 1 , H 2 , H 3 , H 4 , and H 5 , and the number of openings to obtain a viscosity region with good efficiency, under the above-mentioned flow conditions. 
     As described above, in the vacuum envelope and the vacuum envelope evacuating method according to the present invention, an opening, which allows gas at high temperature to flow through the envelope, is previously formed and the inside of the envelope is effectively baked before evacuation to oust the residual gas. Hence, the remaining gas is effectively exhausted in the post evacuation steps so that the narrow space can be brought to a high vacuum state in a relatively short time. 
     Moreover, the vacuum envelope can be more small-sized by sealing the evacuation chamber with a chipless cover or by omitting the getter room. 
     In the flat display panel employing field emission elements, the amount of gas remaining in the vacuum envelope largely depends on the product serviceable life and the quality. However, in spite of such a problem, the second embodiment of the present invention, a small, slim vacuum envelope can be fabricated by omitting the getter room.