Gas display panel and method of making same
United States Patent 1191 Durand [451 Apr. 22, 1975 [75] Inventor: Robert D. Durand, Poughkeepsie,  
 [73] Assignee: International Business Machines Corporation, Armonk, NY.  
  221 Filed: June 25,1973  
  21 Appl. No.: 372,873  
 [52] US. Cl 313/220; 316/17 [51] Int. Cl HOlj 17/16; HOlj 9/26 [58] Field of Search 316/17, 18, 19. 20; 313/220, 109.5, 484, 493; 315/169 TV, 169 R; 220/21 R [56] References Cited UNITED STATES PATENTS 2,967.965 1/1961 Schwartz 313/484 3.646.382 2/1972 Gocdc ct al. 313/1095 X 3.778.126 12/1973 Wilson 316/20 Primary E.\&#39;un1inerRoy Lake Assistant E.raminer.lames W. Davie Attorney, Agent, or Firm-Edward S. Gershuny [57] ABSTRACT The improved gas display panel is made up of four layers. The two inner layers constitute the active display device and are constructed utilizing thin sheets of glass in a known manner. The two outer layers are comprised of relatively heavy glass plates of noncritical thickness and flatness that are appropriately spaced from the inner plates. Each of the inner plates preferably has at least one hole passing through it outside of the viewing area of the display so that gas pressures within the three gaps of the four-layer sandwich will be equal and deflection of either of the outer plates will not cause deflection of the inner plates.  
 12 Claims, 1 Drawing Figure GAS DISPLAY PANEL AND METHOD OF MAKING SAME BACKGROUND OF THE INVENTION This invention relates to an improved structure for a gas display panel and to a method of manufacturing same. More particularly, the invention provides a strong structure wherein the need or desirability of center spacers is completely eliminated.  
  U.S. patent application Ser. No. 214,348, filed Dec. 20, 1971 for Gas Panel Fabrication by P. H. Haberland et al, assigned to the assignee of the present application, discloses a method of making a gas panel which method is typical of the type which the present is designed to improve. In accordance with the method described in said application, front and back panel subassemblies are first constructed having conductors geometrically arranged according to the pattern desired for the eventual display. The front and back panels are connected together by means of a seal which forms a chamber for receiving the display gas. The panels are spaced apart by a suitable spacer. After the partially completed gas panel has been assembled, it is necessary to bake-out and back-fill the assembly. To accomplish this, a hole is provided in the back plate which lies outside of the viewing area of the panel but which communicates with the inner chamber. A glass tube is fitted into the hole and sealed to the assembly. The inner chamber is first evacuated and then filled with display gas through the tube which is then sealed off.  
  Improvements to the above-described method of manufacturing a gas display panel are described in copending U.S. patent application Ser. No. 214,174, filed Dec. 30, 1971 for Gas Display Structure Without Exhaust Tube Structure by D. M. Wilson and in copending U.S. patent application Ser. No. 35 1.672, filed Apr. l6, I973, for Method of Making Tubeless Gas Panel by M. Beckerman et al, both of said applications being assigned to the assignee of the present application. Each of these applications describes a method for evacuating and back-filling the inner chamber of the display panels without using the tube described in said Haberland application; Referring particularly to the method described by Wilson, the front and back panels of the display, with sealing glass interposed therebetween, are placed into a vacuum oven which is first evacuated and then filled with display gas. While the oven is filled with display gas (said gas also filling the unsealed inner chamber of the display panel) heat is applied so that the glass sealing material fuses with the plates to form a containing envelope around the gas surrounded by these elements.  
 In the improved process described by Wilson, the  
  evacuating, back-filling and heating steps are all performed within the oven enclosure so there is never a differential in the pressure exerted upon the glass parts of the panel assembly. Consequently, the thickness of ent pressures. Another reason that center spacers might be required in prior art display panels is that such panels are frequently subjected to pressures substantially in excess of atmospheric pressures. For example, it is not unusual for one person who is viewing a display to inadvertently exert pressure upon the display panel while pointing out a feature of the display to another viewer. Of course, it is undesirable to use center spacers, primarily because they may interfere with panel operation and/or viewing because there can be no display in the areas occupied by the spacers. Also, panels produced by the prior art methods are quite fragile and must be handled with extreme care.  
 OBJECTS OF THE INVENTION There are two primary objects of this invention:  
 1. to provide a gas display panel which, after manufacture, is substantially less fragile than prior art panels; and  
 2. to provide a gas display panel which, irrespective of size, does not require center spacers.  
 SUMMARY OF THE INVENTION The above and other objects are realized in accordance with a preferred embodiment of the invention by providing a gas display panel which comprises four plates of glass. The two inner plates comprise the active display device and may be manufactured in accordance with techniques described in the references referred to above. The two outer plates each comprise a relatively thick sheet of glass each of which is sealed to one of the two inner plates with a spaced therebetween. In preferred embodiments of the invention, each of the two inner plates has at least one hole, outside of the viewing area, passing through it so that the three chambers formed by the four-layer panel are interconnected. The chambers are all filled with display gas. With this arrangement, even if an accident were to temporarily distort one of the protective outer plates, the pressure within the three chambers would remain equal and the distortion would not be transmmitted to transmitted of the inner plates.  
  A primary advantage of this invention is that the display panels provided are less fragile than those in the prior art and can withstand relatively rough handling.  
  Another significant advantage is that even realtively large display panels will not require the use of center spacers in order to maintain the critical gap between front and back plates.  
  Yet another advantage of the invention is that the in advertent application of extraordinary pressures to the display panel will not distort the active parts of the device or materially effect the operation of same.  
  The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of a preferred embodiment thereof as illustrated in the accompanying draw- DESCRIPTION OF THE DRAWING The single FIGURE is a cross-sectional view of a gas display panel manufactured in accordance with the invention.  
 DETAILED DESCRIPTION It is to be understood in connection with the inventive method described herein that it is a subset or step of the overall fabrication process where most of the steps performed in the process can be in accordance with prior art techniques using known materials and procedures. The differences between the prior art and the present method will be pointed out in detail below. Reference may be had to the aforementioned copending applications, which are to be regarded as being incorporated herein by the previous references, for details which are not necessary for an understanding of the method of the invention. Also, with respect to the inventive panel itself, only those aspects which differ from the prior art will be described in detail herein. Reference may be had to said copending applications for additional structural details.  
  The FIGURE shows a display panel constructed in accordance with a preferred embodiment of this invention. The panel comprises four plates of glass 1, 2, 3, 4 spaced apart by spacers 5 and sealed together by frames of sealing glass 6, each frame of sealing glass extending completely around the periphery of the glass plates to provide a gas-tight seal. Holes 7 and 8 through the inner plates 2 and 3, respectively, interconnect the three chambers 9, 10 and 11 so that. irrespective of any deformation of the outer plates 1 and 4, the same gas pressure is maintained on both sides of each of the inner plates 2 and 3.  
  The two inner plates 2 and 3 comprise the active display device and are identical to the front&#34; and back plates described in said copending applications. Each of plates 2 and 3 comprises a glass plate with appropriate configurations of conductive and dielectric materials deposited upon the surface of the plate which faces the inner chamber 10. Conductive strips 12 on plate 2 are orthogonal to conductive strips 13 on plate 3.  
  The outer plates 1 and 4 are passive&#34; in the sense that they do not directly contribute to the displaying of information on the screen, their primary function being to protect the inner plates. Each of the outer passive plates is preferably made of a relatively thick, sturdy sheet of glass which may be of the same composition as the glass which forms the substrate for the inner plates 2 and 3. Examples of appropriate glass compositions are given in the copending applications referred to above.  
  Of course, it is not necessary that plates 1 and 4 be heavier than plates 2 and 3 in order to realize substantial advantages with this invention. In fact, in applications where the added durability of heavier passive plates is not required, the use of identical glass plates for the passive and active parts of the display panel will be more economical because many pieces of glass which would be unsuitable for use in plates 2 or 3 could still be perfectly suitable for use as one of the passive plates 1 or 4. For example, a plate of glass which is lightly scratched on both sides or which contains one or more chips or deep scratches would generally not be used for either of the active plates 2 and 3 but, as long as the defects were not significant enough to materially effect viewing of a display, such plates could be used for the passive plates. Another alternative would be to use a relatively heavy sheet of glass for the front passive plate (that is, the plate which is exposed when the entire display device is constructed) and to use a thinner plate in the back of the panel. It should also be noted that glass plates which would be unacceptable for use as either of the active plates 2 or 3&#39;because they do not have the required flatness (generally 50 millionths or better) might still be perfectly acceptable for use as one of the passive plates 1 or 4.  
  The basic steps in manufacturing the display panel in accordance with a preferred embodiment are as follows:  
  1. Two plates of glass (2 and 3 in the drawing) which may be soda-lime-silica glass are cut to the appropriate dimensions according to the desired size of the gas panel. A hole is drilled in each plate outside of the viewing area. The plates are overlapped and the overlap area serves as the display or storage portion of the panel.  
  2. A first thin film of chromium approximately 1,000 Angstroms thick is deposited on one side of each glass plate. A second thin film of copper approximately 10,000 Angstroms thick is deposited on the first thin film of each glass plate. A third thin film composed of chromium approximately 1,000 Angstroms thick is deposited on the second thin film of each glass plate. The deposition of these thin films to form a laminate preferably is done by vacuum metalization techniques. The laminate terminates a given distance from the edges of the glass plates for reasons explained later.  
  3. A photolithographic process is used to convert the laminate into a plurality of parallel lines which serve as electrical conductors. A liquid photoresist material is applied, preferably by roller, over the outer thin film of chromium and baked dry. The photoresist material is exposed to a light pattern of art work having the desired shape of parallel lines to be formed. After the exposure of the photoresist material is made, the two glass plates are immersed in a developer until the exposed resist material is removed. The unexposed areas of the photoresist remain undisturbed. Each glass plate is cleaned and then immersed in a solution which etches away the chromium-copper-chromium laminate from regions not protected by the resist material. This etching process leaves a plurality of parallel lines with each line being composed of a chromium-copperchromium laminate having an outer coating of unexposed resist. This resist is exposed and placed in a developer until it is removed. The resulting parallel lines terminate a given distance from the edges of the glass plates.  
  4. The two glass plates are heated in a forming gas atmosphere composed of percent nitrogen and 10 percent hydrogen&#39;and water vapor. The outer surface of the third thin film composed of chromium thereby is oxidized, and the chromium oxide surface prevents a subsequent coating operation from dissolving the laminated parallel lines. This step is referred to as passivation since it renders the laminated parellel lines passive during a subsequent dielectric coating operation.  
  5. A glass frit is applied over the electrically conductive parallel lines of each glass plate. The glass frit preferably is a lead glass. The glass frit is applied to a uniform depth over each glass plate by precision spraying. The two glass plates are then fired in an oven to a temperature sufficient to reflow the glass frit whereby a glass coating-covers completely the parallel lines. The parallel lines terminate a given distance from the edges of each glass plate, as pointed out above, and since the lead glass coating extends beyond the ends of these parallel lines, it follows that the ends, sides and top of each one of the parallel lines is coated over by the lead glass coating. The covering of the ends of these parallel lines is done to prevent a reaction in subsequent firing steps of the process from attacking and destroying end regions of these lines. The lead glass coating serves also to prevent the electrically conductive parallel lines from directly contacting the illuminable gas when such gas is inserted in the completed gas panel. The lead glass coating serves as a dielectric material which collects a wall charge when the parallel lines are used subsequently as electrodes for operating the gas panel. Furthermore, the lead glass coating provides mechanical strength and support for the thin laminated conductors which enables them to withstand thermal and mechanical stress and shock during and after the fabrication process.  
  6. Two plates of glass (1 and 4 in the drawing) which may also be soda-lime-silica glass are cut to the appropriate dimensions according to the desired size of the gas panel.  
  7. The panel is assembled in an unjoined state and placed in an air-tight vacuum-oven. Plate 4 is placed in the oven first and unfused glass sealant in picture frame strips&#34; is placed around the periphery thereof. The sealant may be pre-shaped lead-borosilicate glass rods or powdered glass frit in a viscous binder. Spacer rods 5 are placed on the surface of the plate near and within the sealant border. Plate 3 with its interior&#39; face (the coated surface) up is placed on top of the border sealant. Sealant and spacer rods are placed around the periphery of the upper face of plate 3 and plate 2, with its &#34;interior&#34; face down is then placed on top of the border sealant with the metalization on plates 2 and 3 being orthogonally aligned. The placement of sealant and spacer rods is repeated around the periphery of plate 2 and plate 1 is then placed thereupon.  
  8. The vacuum oven is first evacuated and then filled with an illuminable gas to a pressure of approximately 600-700 torrs. The illuminable gas may be any one or a combination of several well known gases used for this purpose. One suitable combination is an illuminable gas composed of 99.9 percent neon and 0.1 percent argon. Because the display panel has not yet been sealed, its chambers 9, and 11 will now contain the illuminable gas. The oven is then heated from room temperature (approximately 25C) to approximately 500C at a rate of l3C per minute to establish fusion of the sealant to the plates of the panel. The oven is then cooled back to room temperature at the same rate. (The softening point of the glass plate and of the spacers is well in excess of 500C.)  
  9. The dielectric coating and the outer or third thin film composed of chromium are removed in the end regions of the parallel lines on each plate so that electrical connections can be made to the exposed copper lands. The dielectric coating may be removed by immersion in an etching solution containing hydrochloric acid, and the third layer composed of chromium may be removed by immersion in a solution of potassium ferricyanide.  
  Although. in the above description, reference was made to a particular technique for evacuating and back-filling the display panel, it will be recognized that any convenient method may be used.  
  Those skilled in the art will also recognize that all four plates of the panel need not be assembled together in a single step. For example, the inner plates 2 and 3 which comprise the active portion of the display panel could be manufactured and assembled in one of the manners known in the prior art and the additional passive plates could be added at a later time. This alternative would be of particular value if one, not having been aware of the invention described herein, had already manufactured gas display panels and wished to improve them by adding the passive plates.  
  In yet another embodiment of the invention the holes 7 and 8 in the active plates 2 and 3 could be eliminated. With this alternative some of the advantages of the invention will be lost (for example, deformation of either of the passive plates could result in some deformation of one or both of the active plates) but, again. could be of particular value to one who had already manufactured his display panels in accordance with the prior art and wished to mdoify them in accordance with this invention. If this alternative were to be used, it would be highly preferable that the original evacuation and backfilling of the inner chamber 10 between the active plates were done utilizing a method wherein tubulation is not necessary. This is because the presence of a tube protruding from plate 2 or 3 would be a hindrance to the step of attaching the passive plate thereto.  
  Still another alternative to the invention which would be most particularly advantageous when used in conjunction with the alternative discussed in the previous paragraph would be to provide one or more holes 14 through one or both of the outer passive plates 1 and/or 4 so that if the outer plate were deformed the deformation would be less likely to be passed on to one or both of the inner active plates (of course, this would negate the advantage of preventing ambient atmospheric pressure, about 750 torrs, from impinging upon the active plates). Or, alternatively, chambers 9 and 11 could be back-filled to a pressure which is somewhat less (for example, about torrs less) than the pressure within the central chamber 10. This differential should be made small enough so that the higher pressure within chamber 10 will not cause either of plates 2 and 3 to distort outwardly, while it provides a cushion which will tend to lessen the effect that inward deformation of plates 1 and 4 would have on plates 2 and 3.  
  While the invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that the above and other changes in form and details may be made therein without departing from the spirit and scope of the invention.  
 What is claimed is:  
  1. In a method of fabricating a gas panel, said method including the step of sealing two active glass plates together spaced apart a given distance to form a center chamber therebetween, each of said active glass plates having dielectric coated conductive electrodes on the inner surface thereof, and the step of filling said chamber with an illuminable gas; the improvement comprising the additional step of:  
 sealing to the exterior face of each of said active glass plates a passive plate which does not directly contribute to the displaying of information on said gas panel, both of said passive plates being spaced apart a given distance from the adjacent active plate to form two outer chambers. 2. The method of claim 1 wherein all four of said plates are sealed together in one step at the same time.  
  3. The method of claim 1 further including the additional step of filling said outer chambers with a gas. 4. The method of claim 1 further including the step of placing at least one hole in one of said passive plates to interconnect one of said outer chambers with the ambient atmosphere. 5. The method of claim 4 further including the step of placing at least one hole in the other of said passive plates to interconnect the other of said outer chambers with the ambient atmosphere. 6. The method of claim 1 further including the step of placing at least one hole in each of said active plates to interconnect said center chamber with each of said outer chambers; and wherein all of said chambers are filled with said illuminable gas in said filling step. 7. The method of claim 6 wherein the holes placed in said active plates are placed outside of the area that will&#39;be utilized for displaying data. 8. The method of claim 7 wherein all four of said plates are sealed together in one step at the same time.  
  9. In a gas display panel which comprises two active glass plates sealed together in a spaced relationship with a center chamber therebetween, each of said active glass plates having dielectric coated conductive electrodes on the inner surface thereof, said center chamber being filled with an illuminable gas, the improvement comprising:  
 a first passive plate sealed to one of said active plates in a spaced relationship thereto, forming a first outer chamber therebetween; and  
 a second passive plate sealed to the other of said active plates in a spaced relationship thereto, forming a second outer chamber therebetween,  
 both of said passive plates being free of electrical connections that would directly affect the displaying of information on said gas display panel.  
 10. The improved gas display panel of claim 9 wherein each of said outer chambers is filled with a gas.  
  11. The improved gas display panel of claim 10 wherein:  
 each of said active plates has at least one hole passing therethrough to connect each of said outer chambers to said center chamber;  
 all three of said chambers being filled with said illuminable gas.  
 12. The improved gas display panel of claim 11 wherein:  
 each hole through an active plate is so located as to be outside of the area that will be utilized for displaying data.