EP0105408B1

EP0105408B1 - Electroluminescent display

Info

Publication number: EP0105408B1
Application number: EP19830109358
Authority: EP
Inventors: Henry T. Hidler; Lawrence L. Hope; Ernst A. Davey; Martin P. Schrank
Original assignee: GTE Products Corp
Current assignee: Osram Sylvania Inc
Priority date: 1982-09-30
Filing date: 1983-09-20
Publication date: 1988-01-07
Also published as: JPH0380314B2; JPS59131975A; EP0105408A1; CA1229905A; DE3375273D1

Description

Technical Field

The present invention relates in general to thin film, multi-layer, electroluminescent display devices, and more particularly, to an improved hermetic seal for use with such electroluminescent display devices.

Background Art

In the construction of thin film electroluminescent display devices, there is a requirement for a hermetic seal to protect the device against contamination. Water vapor is in particular damaging to the thin films that are deposited in constructing the display device. Any water or humidity penetration of the hermetic seal causes rapid degradation in display performance. Even relatively slow water penetration substantially shortens the useful life of the display. This hermetic seal is typically provided between the glass substrate upon which the thin films are deposited, and a cover sheet usually in the form of a protective glass plate.
At the present time, there are two types of hermetic seals that are predomonantly used; one an epoxy seal and the other a glass frit seal. The epoxy hermetic seal particularly has problems under excess humidity conditons. Present epoxy seals are effective under high humidity conditions for only up to periods of seven hundred hours of operation. The glass frit seal does not suffer from degradation under high humidity conditions, but it does break down at higher temperatures.
Independent claims 1 and 11 are based in their preamble part on GB-A-2 020 684 disclosing an electroluminescent display and a methodforform- ing a hermetic seal between a base substrate and a cover sheet of such an electroluminescent display. The known display includes a hermetic seal in the form of an adhesive, for example a photo-curing resin bonding a counter-substrate and a plane substrate. Moreover, the hermetic seal comprises a protective liquid which is filled in the space between the substrates and the pin holes of the dielectric layers. The protective liquid prevents the electroluminescent panel from being damaged by increasing the small thermally-damaged area. The protective liquid also shuts any moisture out of the electroluminescent panel.
However, the display known from said reference suffers from the drawback of a relatively complicated arrangement due to the necessity of the introduction of the protective liquid. Moreover, the epoxy seal used with the known display incurs problems under conditions of excess humidity. Thus, it is possible that moisture diffuses through the adhesive and causes rapid degradation in the display performance and therefore, the additional protective liquid has to be provided with the known display.
Reference IBM Technical Disclosure Bulletin, Vol. 23, No. 11, April 1981, page 5111 discloses a seal for a liquid crystal cell. The disclosed seal requires a picture frame of a non-wettable metal to be deposited on the coated glass and substrate and furthermore, a wettable metal to be deposited on the coated glass and substrate within the twin-lines of the non-wettable metal and finally, a solder alloy to be deposited on top of the wettable metal. Therefore, the formation of the seal of the display known from said latter reference involves the exact positioning of the various components of the seal with respect to each other. Furthermore, the portion of the seal on the coated glass has an identical construction to the matching portion of the seal on the substrate. Thus, said latter reference only discloses applying the solder directly to a wettable metal. Moreover, it merely shows the use of twin-lines of non-wettable metal to prevent shorting caused by spillage of solder on to the electrodes. Also, because liquid crystal cells require uniform spacing between the two electrode-bearing sheets of the devices, the thicknesses of the materials used to form the seal of the display known from said latter reference would require careful control.
Therefore, the IBM reference also discloses a complicated seal arrangement and a complicated and expensive method requiring the build-up of metallic layers.
It is an object underlying the present invention to provide an electroluminescent display according to the preamble part of claim 1 and a method for forming a hermetic seal for such a display according to the preamble part of claim 11 which substantially ensures total protection against penetration of water, vapour and other contaminants without necessitating a complicated structure and fabrication.
The solution of this object is achieved by the features of claims 1 and 11, respectively.
Advantageously, the present invention provides an economical method for easily manufacturing a reliable seal for a thin film display device which is compatible with thin film processes. The seal of the display according to the present invention provides substantially total protection against penetration of water, vapour and other contaminants even at high temperatures without requiring a complicated arrangement and without requiring the provision of an additional protective liquid.
Moreover, an advantage of the present invention is to provide an improved hermetic seal for an electroluminescent display device having an increased operating life and having substantially total resistance to damasge by humidity. This permits the use of the electroluminescent display device outdoors and in other harsh environments.

Brief Description of the Drawings

Fig. 1 is a plan view of an electroluminescent display constructed in accordance with the present invention and employing the hermetic seal thereof;
Fig. 2 is a cross-sectional view showing the constructon of the display including the forming of the hermetic seal; and
Fig. 3 is a fragmentary rear view showing further details of the hermetic seal.

Best Mode for Carrying Out the Invention

For a better understanding of the present inven- tin, together with other and further objects, advantages and capabilities thereof, reference is made to the following disclosure and appended claims taken in conjunction with the above- described drawings.
There is now described in detail a sealing technique and associated method of fabrication for providing a true hermetic seal between the base substrate and cover sheet or cover plate of an electroluminescent display device. The sealing is provided by means of a eutectic (solder) metal, or combinations of metal, preferably disposed in a picture frame configuration about the periphery of the display substrate. The cover sheet, which may be glass, ceramic or other material, is typically a flat element lying in a plane lying in parallel to the substrate and separated from the substrate by the aforementioned picture frame seal. The cover sheet may carry leads, drive chips, or other ancillary components. In combination with the metal seal is a thin film dielectric insulating layer which covers the area on the substrate and cover sheet over which the electrical leads pass and upon which the metallic seal is formed. This insulating layer extends beyond the seal area to provide a safety margin.
With reference to the drawings, FIG. 1 is a plan view of an electroluminescent display constructed in accordance with the present invention and employing the hermetic seal of the invention. FIG. 2 is a cross-sectional view showing the details of the hermetic seal and other thin films associated with the display device. Finally, FIG. 3 is a fragmentary rear view showing further details of the hermetic seal.
In the illustrated embodiment the different layers that are shown may be deposited by known techniques such as by vapor deposition, thermal evaporation, electron beam evaporation or sputtering. Because the deposition process is basically known, it is not discussed in any detail herein. Also, because the formation of the basic thin films such as those forming the phosphor is also well known, the process involving the deposition of these thin films is not described in detail herein.
The thin film, multi-layer electroluminescent device basically comprises, from front to back, a glass substrate 20, a transparent front electrode or conductor pattern 22, and an aluminum back electrode pattern 24. The front electrode pattern 22 and the back electrode pattern 24 are formed by known deposition techniques with the employment of the proper mask. FIG. 1 shows the deposited patterns for the transparent front electrode pattern 22 and the aluminum back electrode pattern 24.
The back electrode pattern 24 may be formed by the deposition of a metal such as aluminum. The transparent front electrode pattern may be formed by the deposition of a doped tin oxide. Alterntively, the front electrode pattern may also be constructed by depositing an indium-tin oxide.
There is provided between the front and back electrodes, a layer 32 that includes at the very least, the phosphor. FIG. 2 shows the layer 32 as actually being formed, in one embodiment, by four separate layers, including the phosphor layer 34, the dark field layer 40, and dielectric layers 36 and 38. The layer 36 is a dielectric layer that functions as a capacitive ballast layer. This may be formed of a material such as yttrium oxide. The phosphor layer 34 may be formed of a zinc sulfide usually doped with manganese. Another material that may be used as a phosphor is zinc selenite. The layer 38 is usually formed of silicon nitride. Layer 38 is a dielectric insulating layer. The dark field layer 40 may consist of a cermet of chromium oxide and chromium.
In the display device there is also provided a plurality of aluminum lead-ins 50 for providing electrical connection to the segments 46 of the back electrode pattern 24. FIG. 1 shows the aluminum lead-ins 50 extending to the transparent electrode terminal pads 23.
With reference to FIG. 1, it is noted there are provided seven such aluminum lead-ins 50 associated respectively with the seven segments 46 of the single digit display. In addition, there are also two additional aluminum lead-ins that couple from the front electrode pattern 22. All of these aluminum lead-ins are shown in FIG. 1, coming out at the pads 23.
Fig. 2 shows the additional layers that form the hermetic seal of the present invention. These layers include a first layer which is an insulating layer preferably of silicon nitride in combination with a eutectic sealing material. These are illustrated in FIG. 2 as respective layers 60 and 62. The basic sealing function is provided by the eutectic mixture or metallic sealing element. The silicon nitride layer 60 functions primarily as an insulating layer so as to insulate the metallic sealing layer 62 from any electrical leads that pass beneath it. In this regard note FIG. 3 and leads 50 that pass under the sealing area. Although, FIG. 2 illustrates such aluminum leads as coupling only from the back electrode 24, it is understood that such leads may also be in the form of conductive thin films associated with the cover sheet or a back glass 70. Also note, as illustrated in FIG. 1, that the sealing area is in a picture frame configuration so as to provide a total hermetic seal about the entire display area.
The silicon nitride layer 60 may be deposited using substantially the same equipment as is used for depositing the other thin film layers forming the electroluminescent device. Alternatively, the silicon nitride layer 60 may be deposited using separate equipment. This insulating layer covers the area on the substrate or the cover sheet over which the electrical leads pass and upon which the metallic seal lies. The insulating layer, as illustrated in FIG. 3 extends beyond the seal layer to provide a safety margin.
After the conductive thin film leads associated with the base substrate and/or cover sheet are covered by the insulating film in the seal area, the eutectic metallic solder layer 62 is applied to the seal area on both the substrate and cover sheet in substantially identically shaped picture frame configurations, such as illustrated in FIG. 1. The application of the layer 62 may be by known techniques such as by flashing or other means of deposition. These separately deposited picture frame configurations of the layer 62 are then mated and heated so that the solder material meets and flows together to form a single seal between the base substrate and cover sheet or protective back glass.
In an alternative technique for forming the hermetic seal, the eutectic sealing material may be applied in a paste form such as by depositing a bead in a picture frame configuration. In this instance, the device is then subjected to heat for driving off binders or other foreign materials associated with the paste. Also, the eutectic sealing material is one that is chosen for good adherence to the insulating film.
In practicing the method of forming the hermetic seal of this invention, it is preferred that the fabrication of the electroluminescent device be carried out in a dry box or other controlled environment. When using a dry box the parts are inserted, mated, and heated to reflow the seal material. If required, an additional layer of the metallic seal material may be used between the sealing elements on the substrate and cover sheet.
In a preferred embodiment described herein, the layer 62 may be formed of a tin solder and the insulating layer 60 is preferably silicon nitride. The insulating layer 60 may alternatively be formed of silicon oxide or silicon dioxide.
While there have been shown and described what are at present considered to be the preferred embodiments of the invention, it will be apparent to those skilled in the art that various changes and modifications can be made herein without departing from the scope of the invention as defined by the appended claims.

Claims

1. An electroluminescent display comprising a base substrate (20) upon which multiple thin film layers (22, 24, 32, 34, 36, 38, 40) are formed including electrically conductive thin film leads (50) and a protective cover sheet (70) disposed over the thin film layers (22, 24, 32, 34, 36, 38, 40) and comprising a hermetic seal means (60, 62) for joining the cover sheet (70) to the base substrate (20) being characterized in that said hermetic seal means comprises an electrically insulative thin film layer (60) sealingly disposed over the conductive thin film leads (50), and a metallic solder layer (62) over the insulative thin film layer (60) thereby providing a bond between the cover sheet (70) and the base substrate (20) without causing shorting between the leads (50).

2. An electroluminescent display according to claim 1, being characterized in that said insulative thin film layer (60) and said metallic solder layer (62) are disposed in a picture frame configuration.

3. An electroluminescent display according to claim 1, being characterized in that said protective cover sheet (70) comprises a glass plate.

4. An electroluminescent display according to claim 1, being characterized in that said protective cover sheet (70) comprises a ceramic plate.

5. An electroluminescent display according to claim 1, being characterized in that said metallic solder layer (62) comprises tin.

6. An electroluminescent display according to claim 1, being characterized in that the electrically insulative thin film layer (60) is comprised of silicon nitride.

7. An electroluminescent display according to claim 1, being characterized in that said electrically insulative thin film layer (60) is comprised of silicon oxide.

8. An electroluminescent display according to claim 1, being characterized in that said electrically insulative thin film layer (60) is comprised of silicon dioxide.

9. An electroluminescent display according to claim 1, being characterized in that the area covered by the insulative thin film layer (60) is greater than the metallic solder layer to provide a safety margin to assure no shorting of the leads (50).

10. An electroluminescent display according to claim 1, being characterized in that said metallic solder layer (62) is a eutectic mixture.

11. A method of forming a hermetic seal (60,62) between a base substrate (20) and a protective cover sheet (70) of an electroluminescent display, the method comprising the step of forming multiple thin film layers (22, 24, 32, 34, 36, 38, 40) upon said base substrate (20), said thin film layers including electrically conductive thin film leads (50), the step of disposing a protective cover sheet (70) over the thin film layers and the step of forming a hermetic seal between the base substrate (20) and the protective cover sheet (70) being characterized in that the method step of forming said hermetic seal (60, 62) includes the step of sealingly depositing an electrically insulative layer (60) over the conductive thin film leads (50) and depositing a metallicsolder layer (62) over the insulative layer (60) and the protective cover sheet (70).

12. The method according to claim 11, being characterized in the step of mating the base substrate (20) and the protective cover sheet (70) with the deposited solder layers respectively thereof aligned and applying heat to join the base substrate (20) and the protective cover sheet (70) and form the hermetic seal therebetween.

13. The method according to claim 12, being characterized in the step of depositing the solder layers as a paste.