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(11) | EP 0 000 263 A1 |
| (12) | EUROPEAN PATENT APPLICATION |
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| (54) | Gaseous discharge display device |
| (57) A high resolution gaseous discharge display and/or memory device (21) comprises a
panel array of bistable charge storage areas designated gaseous discharge cells or
sites, each cell having an associated pair of coordinate orthogonal conductors (23A-23N
and 25A-25N) defining the cell walls which, when appropriately energized, produce
a confined gaseous discharge in the selected sites. The conductors (23A-23N and 25A-25N)
are insulated from direct contact with the gas by a dielectric insulator (33,35,39,41)
the dielectric insulator (39,41) being composed of a layer of refractory material
having high secondary emission characteristics such as a Group IIA oxide doped with
a Group IA element to prevent degradation of the dielectric during operation, to increase
the static margin of the panel and improve the stability of the maximum and minimum
sustain voltages, thereby providing stable operating voltages and extending the life
of the gaseous discharge display panel. Other embodiments of the dielectric insulator
utilize additional small concentrations of elements of Groups VIII or VIB with the
above concentration. |
[0002] Plasma or gaseous discharge display and/or storage apparatus have certain desirable characteristics such as small size, thin flat display package, relatively low power requirements and inherent memory capability which render them particularly suitable for display apparatus. One example of such known gaseous discharge devices is disclosed in U.S. Patent 3,559,190, "Gaseous Display and Memory Apparatus", patented January 26, 1971 by Donald L. Bitzer et al and assigned to the University of Illinois.- Such panels, designated a.c. gas panels, may include an inner glass layer of physically isolated cells or comprise an open panel configuration of electrically insulated but not physically isolated gas cells. In the open panel configuration, which represents the preferred embodiment of the present invention, a pair of glass plates having dielectrically coated conductor arrays formed thereon are sealed with the conductors in substantially orthogonal relationship. When appropriate drive signals are applied to selected pairs or groups of conductors, the signals are capacitively coupled to the gas through the dielectric. When these signals exceed the breakdown voltage of the gas, the gas discharges in the selected area, and the resulting charge particles, ions and electrons, are attracted to the wall having a potential opposite the polarity of the particle. This wall charge potential opposes the drive signals which produce the discharge, rapidly extinguishing the discharge and assisting the breakdown of the gas in the next alternation. Each discharge produces light emission from the selected cell or cells, and by operating at a relatively high frequency in the order of 30-40 kilocycles, a flicker-free display is provided. After initial breakdown, the discharge condition is maintained in selected cells by application of a lower potential designated the sustain signal which, combined with the wall charge, causes the selected cells to be reignited and extinguished continuously at the applied frequency to maintain a continuous display.
[0003] The capacitance of the dielectric layer is determined by the thickness of the layer, the dielectric constant of the material and the geometry of the drive conductors. The dielectric material must be an insulator having sufficient dielectric strength to withstand the voltage produced by the wall charge and the externally applied potential. The dielectric surface should be a relatively good emitter of secondary electrons to assist in maintaining the discharge, be transparent or translucent on the display side to transmit the light generated by the discharge for display purposes, and be susceptible to fabrication without reacting with the conductor metallurgy. Finally, the coefficient of expansion of the dielectric should be compatible with that of the glass substrate on which the dielectric layer is formed.
[0004] According to the present invention there is provided gaseous discharge display device in which a plurality of display cells are defined by the intersections of a first set and a second set of parallel conductors carried on a first and a second substrate respectively and arranged orthogonally and in which the conductors are insulated from direct contact with an ionizable gas contained within the cells by a dielectric insulating coating, characterized in that the dielectric insulating coating is an electron emissive substance having refractory properties and is selected from a combination of one Group IA and one Group IIA oxides.
[0005] In one embodiment of the invention, magnesium oxide, a refractory material characterized by a high coefficient of secondary emission, is doped with a Group IA element such as lithium and applied over the entire surface of the dielectric layer. By utilizing magnesium oxide, the secondary electron emission characteristics dominate the electric operating conditions in the gas panel, resulting, as more fully described hereinafter, in gaseous discharge operation with lower operating voltages. Doping the magnesium oxide overcoat with elements of Group IA such as lithium or with lithium and small concentrations of elements of Group VIII (e.g., iron or nickel) or Group VIB (e.g., chromium) results in substantially no change in the maximum and minimum sustain voltage during test or ageing, The lithium concentration, which-may vary from 5 to 40 atomic percent, significantly improves the stability of Vs max. with panel operating time, thereby extending the useful life of the gas panel. The memory margin of the cells is increased by increasing the maximum sustain voltage at a higher rate than that of the minimum sustain voltage. The alternate line ageing problem is eliminated, thereby increasing the panel yield and minimizing rejection of panels with inadequate memory margin.
[0006] In order that the invention may be fully understood a preferred embodiment thereof will now be described with referonct to the accompanying drawings, in which:
Figure 1 is an isometric view of a gaseous discharge panel broken away to illustrate details of the present invention; and
Figure 2 is a top view of the gaseous discharge panel illustrated in Figure 1.
[0007] Referring now to the drawings and more particularly to Figure 1 thereof, there is illustrated a gas panel 21 comprising a plurality of individual gas cells or sites defined by the intersection of vertical drive lines 23A-23N and horizontal drive lines 25A-25N. The structure of the preferred embodiment as shown in the drawings is enlarged, although not to scale, for purposes of illustration; however, the physical and electrical parameters are fully described in detail hereinafter. While only the viewing portion of the display panel is illustrated in the interest of clarity, it will be appreciated that in practice the drive conductors extend beyond the viewing area for interconnection to the driving signal source.
[0008] The gas panel 21 includes an ionizable gas such as a mixture of neon and argon within a sealed structure, the vertical and horizontal conductor arrays being formed on associate glass plates and disposed in orthogonal relationship on opposite sides of the structure. Gas cells within the panel are selectively ionized during a write operation by applying to the associated conductors coincident potentials having a magnitude sufficient when algebraically added to exceed the breakdown voltage VB. In the preferred embodiment che control potentials for write, read and erase operations may be square wave a.c. signals. Typical operating potentials for a gaseous discharge panel with nominal deviations using a neon-argon gas mixture are 150 volts for write, 93 to 99 volts for sustain V s max. , and 83 volts for sustain minimum voltage V min. Once the wall charge has been established, the gas cells are maintained in the discharge state by a lower amplitude periodic sustain signal. Any of the selected cells may be extinguished, termed an erase operation, by first reducing the potential difference across the cell by neutralizing the wall charges so that the sustain signal is not adequate to maintain the discharge. By selective write operations, information may be generated and displayed as a sequence of lighted cells or sites in the form of alphanumeric or graphic data, and such information may be regenerated as long as desired by the sustain operation.
[0009] Since the dielectric or its associated overcoat interfaces directly with the gas, it may be considered a gas panel envelope comprising relatively thin or fragile sheets of dielectric material such that a pair of glass substrates 27, 29, front and rear, is employed as support members on opposite sides of the panel. The only requirement for such support members is that they be non-conductive and good insulators, and substantially transparent for display purposes. One-quarter inch thick commercial grade soda-lime-silica glass is utilized in the preferred embodiment.
[0010] Shown also in cutaway is conductor array 25 comprising conductors 25A-25N which are interposed between the glass substrate 27 and associated dielectric member 33. The corresponding configuration for conductor array 23 is illustrated in Figure 2. Conductor arrays 23, 25 may be formed on substrates 27, 29 by a number of well knowr processes such as photoetching, vacuum deposition, stencil screening, etc. Transparent, semi-transparent or opaque conductive material such as tin oxide, gold, aluminium or copper can be used to form the conductor arrays, or alternatively the conductor arrays 23, 25 may be wires or filaments of copper, gold, silver or aluminium or any other conductive metal or material. However, formed in situ conductor arrays are preferred, since they may be more easily and more uniformly deposited on and adhere to the substrates 27, 29. In a preferred embodiment constructed in accordance with the present invention, opaque chrome-copper-chrome conductors are utilized, the copper layer serving as the conductor, the lower layer of chrome providing adhesion to the associated substrate, the upper layer of chrome protecting the copper conductor from attack by the lead-borosilicate insulator during fabrication.
[0011] In the preferred embodiment herein described, dielectric layers 33, 35, layer 33 of which is broken away in Figure 1, are formed in situ directly over conductor arrays 25, 23 respectively of an inorganic material having an expanps coeffiecient closely related to that of the substrate me- bers. One preferred dielectric material, is lead-boros licat solder glass, a material containing a high percentage lead oxide. To fabricate the dielectric, lead-borosi the glass frit is sprayed over the conductor array and to ub- strate placed in an oven where the glass frit is reforwed and monitored to ensure appropriate thickness. Attentively, the dielectric layer could be formed by electron ber evaporation, chemical vapour deposition or other sui ble means. The requirements for the dielectric layer hg been specified, but additionally the surface of the diel tric layers should be electrically homogeneous on a micr copic scale, i.e., should be preferably free from cracks Jubbles crystals, dirt, surface films or any impurity or imperfection.
[0012] Finally, as heretofore described, the problem of degradation occurring on an unprotected dielectric surface during operation of the gas panel resulting from ion bombardment produced variation of the electrical characteristics of individual cells and significantly reduced panel life. The solution utilized in the preferred embodiment was the deposition of a homogeneous layer of a magnesium oxide having a high secondary emission characteristic doped with lithium between the dielectric surface and the gas. This homogeneous layer is formed by co-evaporation of the lithium and magnesium oxides in an evaporation system, the respective proportions of the constituents being determined by the respective evaporation rates. Such evaporations take place in the single evacuated chamber during a single pump-down. Such a layer may comprise between 5 and 40 atomic percent lithium, the layer in the preferred embodiment being 3000 9 or .3 microns thick. Within this range, the minimum sustain voltage V min. increases slightly, while the maximum sustain voltage V max. has a greater increase as the percentage of lithium increases. In one embodiment the minimum sustain voltage with a 10 atomic percent lithium concentration was 84 volts; the maximum sustain voltage was 97 volts, while for MgO alone the maximum and minimum sustain voltages were 90 and 80 volts respectively. In the above described preferred embodiment, the constituent magnesium and lithium oxides were co-evaporated using two separate electron guns to provide better control of the relative concentrations of the two oxides comprising the overcoat layer.
[0013] With respect to material having a high secondary electron emission efficiency, the dominant secondary electron production mechanism is defined as emission from the confining boundaries of the gas, which in the present invention are the dielectric surfaces. The breakdown voltage in a gaseous discharge display panel is determined by the electron amplification in the gas volume defined by the coefficient a and the production of secondary electrons at the confining surfaces or cell walls defined by the coefficient y. For a specified gas mixture, pressure and electrode spacing, a is a monotonically increasing function of the voltage in the ordinary range of panel operation. The secondary electron emission is characterized by a coefficient y, which is a function of the surface material and mode of preparation. Voltage breakdown occurs when the following approximate-relationship is satisfied:
where d is the spacing between electrodes. Consideration of the above equation shows that an increase in y will result in a lower value of a at breakdown, and hence a lower breakdown or panel operating voltage Vb. Vs max. is a function of y while V min. is primarily determined by wall charge. Thus the use of lithium doped magnesium oxide increases Vs max. at a relatively high rate, while Vs min. remains essentially constant or increases at a slower rate to provide increased memory margin. In a gas panel constructed in accordance with the teaching of the present invention, having a lithium magnesium oxide overcoat, a graph of A V vs. the square root of time in terms of hours, the panel tested indicated a deviation of less than one- half volt at 1,000 hours. The fabrication process of the panel involved outgassing the panel plates in a vacuum at 350°C. for one hour and then cooling the panel plates in vacuum to room temperature with the lithium-magnesium oxide film deposited at room temperature. A similar graph of a magnesium oxide coated plate tested under identical conditions indicated a deviation in A V , of about -2.5 volts, a substantial difference in terms of the nominal margin values.
[0014] Referring now to Figure 2, a top view is employed to clarify certain details of the present invention, particularly since only a portion of the panel as shown in cutaway in Figure 1. Two rigid support members or substrates 27 and 29 comprise the exterior members of the display panel, and in a preferred embodiment comprise 1/4" commerical grade soda-lime-silica glass. Formed on the inner walls of the substrate members 27 and 29 are the horizontal and vertical conductor arrays 25, 23 respectively. The conductor sizes and spacing are obviously enlarged in the interest of clarity.
[0015] In typical gas panel configuration, the centre-to-centre conductor spacing in the respective arrays is between 14 and 60 mils using 3-6 mil wide conductors which may be typically 2.5 microns in thickness. Formed directly over tfie conductor arrays 25, 23 are the dielectric layers 33 and 35 respectively which, as previously described, may comprise solder glass such as lead-borosilicate glass containing a high percentage of lead oxide. The dielectric members, being of nonconductive glass, function as insulators and capacitors for their associated conductor arrays. Lead-borosilicate glass dielectric is preferred since it adheres well to other glasses, has a lower reflow temperature than the soda-lime-silicate glass substrates on which it is laid, and has a relatively high viscosity with a minimum of interaction with the metallurgy of the conductor arrays on which it is deposited. The expansion characteristics of the dielectric must be tailored to that of the associated substrate members 27 and 29 to prevent bowing, cracking or distortion of the substrate. As an overlay or a homogeneous film, the dielectric layers 33 nd 35 are more readily formed over the entire surface of the gaseous discharge device rather than cell-by-cell definition.
[0016] The lithium doped MgO overcoating over the associated dielectric layer is shown in Figure 2 as layers 39, 41 which, as previously noted, combine a high secondary electron emission efficiency with a resistance to aging during normal panel operations. As in the dielectric layer with respect to the substrate, the overcoating layers 39 and 41 are required to adhere to the surface of the dielectric layers and remain stable under panel fabrication including the high temperature baking and evacuation processes. A 3000 Angstrom thick coating is used in the preferred embodiment. While the lithium doped magnesium oxide coating in the above described embodiment of the present invention was applied over the entire surface, it will be appreciated that it could be also formed on a site-by-site definition.
[0017] The final parameter in the present invention relates to the gas space or gap 45 between the opposing lithium magnesium oxide surfaces in which the gas is contained. This is a relatively critical parameter of the gas panel, since the intensity of the discharge and the interactions between discharges on adjacent discharge sites are functions of the spacing. While the size of the gap is not shown to scale in the drawings, a spacing of approximately 5 mils is utilized between cell walls in the preferred embodiment. Since a uniform spacing distance must be maintained across the entire panel, suitable spacer means, if needed, could be utilized to maintain this uniform spacing. While the gas is encapsulated in the envelope, additional details regarding sealing of the panel or fabrication details such as the high temperature bakeout, evacuation and backfill steps have been omitted as beyond the scope of the present invention.
[0018] While the invention has been described in terms of a preferred embodiment of lithium doped magnesium oxide, it may also be implemented in other Group IA elements doped with magnesium oxide. It was also indicated that doping of magnesium oxide overcoat with elements of Group VIB and Group VIII results in an improved panel stability during ageing. For example, doping the magnesium oxide coating with 0.1 to 0.5 percent by weight of Chromium (Group VIB element) iron or nickel (Group VIII elements) results, on the other hand, in only a slight increase in the maximum and minimum sustain voltage of both the aged and unaged discharge cells during ageing. In addition, doping the magnesium oxide overcoat with lithium (Group IA element) or with lithium and iron (Group VII elements) results in essentially no change in the maximum and minimum sustain during ageing.
[0019] In summary, doping the magnesium oxide coating of a gas panel with elements of Group IA such as lithium results in essentially no change in the maximum and minimum sustain during ageing. Doping the magnesium oxide with Group VIB and Group VIII results in an improved panel stability during ageing. For a given gas pressure, the incorporation of lithium into MgO causes the maximum sustain voltage to increase while the minimum sustain voltages increase, if any, is only nominal, thereby enhancing, the panel margin.
1. A gaseous discharge display device in which a plurality of display cells are defined
by the intersections of a first set and a second set of parallel conductors carried
on a first and a second substrate respectively and arranged orthogonally and in which
the conductors are insulated from direct contact with an ionizable gas contained within
the cells by a dielectric insulating coating, characterized in that the dielectric
insulating coating is an electron emissive substance having refractory properties
and is selected from a combination of one Group IA and one Group IIA oxides.
2. A gaseous discharge display device as claimed in claim 1 further characterized
in that the Group IA oxide is lithium oxide.
3. A gaseous discharge display device as claimed in claim 1 or claim 2 further characterized
in that the Group IIA oxide is magnesium oxide.
4. A gaseous discharge display device as claimed in claim farther characterized in
that the magnesium oxide is doped with a lithium oxide having a concentration of 5
to 40 atomic percent relative to said magnesium oxide.
5. A gaseous discharge display device as claimed in claim 4 further characterized
in that the dielectric insulating coating includes minute amounts of Group VIB elements.
6. A gaseous discharge display device as claimed in claim 5 further characterized
in that the Group VIB elements include chromium.
7. A gaseous discharge display device as claimed in claim 4 further characterized
in that the dielectric insulating coating includes minute amounts of Group VIII elements.
8. A gaseous discharge display device as claimed in claim 7 further characterized
in that the Group VIII elements include iron and nickel.