Electric lamp and discharge devices – With luminescent solid or liquid material – Solid-state type
Reexamination Certificate
1999-07-15
2002-05-14
Patel, Vip (Department: 2879)
Electric lamp and discharge devices
With luminescent solid or liquid material
Solid-state type
C313S503000, C313S506000, C313S512000
Reexamination Certificate
active
06388378
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to thin film structures. In particular the invention concerns insulting films comprising aluminium oxide and titanium oxide for use as dielectric layers in electroluminescent thin film components.
2. Description of Related Art
Thin-film electroluminescent (TFEL) components are used as alternatives to cathode ray tubes, plasma displays, liquid crystal devices and light-emitting diodes (LEDs) for displaying information or data e.g. word or numerals. They are used especially in applications where a wide viewing angle, wide temperature range and a rugged structure are important.
EL display devices generally comprise a substrate, such as a glass plate, a first electrode layer provided on the substrate and a second electrode layer spaced apart from the first electrode layer, a phosphor layer fitted between the electrodes, and at least one dielectric layer provided on each side of the phosphor layer between the phosphor layer and the electrodes.
Aluminium oxide (Al
2
O
3
) has been used extensively in the thin film industry as an insulation material. Various more sophisticated dielectric materials have also been employed and suggested, including strontium titanium and barium tantalum binary oxides and silicon oxynitride (SiON). A particularly advantageous material is disclosed in U.S. patent specification Ser. No. 4,486,487 which describes an insulative film for a thin film structure, comprising alternating layers of aluminium oxide and titanium oxide. For this material, the abbreviation “ATO” will be used in the following. The prior art film consists of 10 to 200 layers of aluminium oxide and titanium oxide, each layer having a thickness of 3 to 1000 Å. According to a preferred embodiment, the ATO film is grown by the Atomic Layer Epitaxy (ALE) taking advantage of the ease of controlling the thickness of the film to be grown by that technique. The ATO dielectric of U.S. Pat. No. 4,486,487 has excellent properties, including a three times better breakdown strength than Al
2
O
3
.
Conventionally, sodium silicate glass (or shortened “soda lime” or “soda” glass) has been used as a substrate for thin film EL components made with ALE. There are, however, a number of disadvantages associated with this material. Thus, for many gas phase vapour deposition methods, in particular ALE, the processing temperatures employed of up to 500 to 600 ° C. are very close to or over the maximum temperature limits at which soda glass can be used. The dimensional stability of soda glass at elevated temperatures is not altogether satisfactory, and the shrinking of the material at high temperatures has to be taken into account during processing of the thin film structures. In some cases soda lime glass is not compatible with other processes. Finally, and importantly, it has been found that some migration of alkali metal, in particular sodium and potassium, ions from the glass substrate to the adjacent layers always takes place. This alkali metal ion migration gives rise to destruction of the thin film structure. In order to prevent the migration thin film stacks are provided with an ion diffusion barrier layer made from metal oxide or nitride.
Many of the problems related to soda glass as a substrate of ALE grown thin films can be avoided by using essentially alkali metal-free glass materials, such as borosilicate glasses. Such materials contain less than 1% alkali metals and the migration of alkali metal ions is negliable for most applications. However, our tests have shown that when a conventional ATO layer having an Al-to-Ti ratio of close to 1 (based on the cumulative thicknesses of the Al
2
O
3
and the TiO
2
layers) is used as insulation in thin film structures supported on alkali metal-free glass substrates, cracking is observed immediately after thin film processing or after subsequent processing. This finding is surprising because for soda glass substrates the layer thicknesses have not been found to be of any particular relevance. Thus, U.S. Pat. No. 4,486,487 suggests that the layer thicknesses should not have any effect on mechanical strength properties.
Cracking is generally caused by internal or external stresses in the film. If the mechanical strength of the film is lower that the force introduced by these stresses, cracking and other types of mechanical failure occur. In many cases thin films made with vacuum deposition techniques exhibit internal stress, either tensile or compressive stress. These are usually characteristic for the deposition method used and even some techniques can give rise to tensile or compressive stress in the films depending on the process parameters used. Also the type of substrate has an effect on film stress.
SUMMARY OF THE INVENTION
It is an object of the present invention to eliminate the problems of the prior art and to provide an insulating film for a thin film structure comprising alternating layers of aluminium oxide and titanium oxide.
It is another object of the present invention to provide a thin film electroluminescent display device, comprising a substrate consisting of an essentially alkali metal-free glass.
These and other objects, together with the advantages thereof over known combination insulator films, which shall become apparent from the specification which follows, are accomplished by the invention as hereinafter described and claimed
According to the present invention, the insulating film of a thin film structure deposited on an essentially alkali metal-free glass substrate is formed by alternating layers of aluminium oxide and titanium oxide while restricting the ratio between the cumulative thicknesses of said titanium oxide and said aluminium oxide layers to less than 0.75.
This film can be incorporated into thin film electroluminescent display devices as an insulating layer between the phosphor layer and the electrode layers.
More specifically, the present insulating films are mainly characterized by what is stated in the characterizing part of claim
1
.
The thin film electroluminescent display devices according to the present invention are characterized by what is stated in the characterizing part of claim
7
.
Considerable advantages are obtained by the present invention. Thus, good resistance is obtained against cracking of ATO film. In spite of the lowered titanium oxide concentrations the film still provides good breakthrough strength. The novel insulating layer structure can be used for a wide range of essentially alkali metal-free glass substrates, including borosilicate and aluminosilicate glasses. It is also compatible with many other processes.
Next, the invention will be examined more closely with the aid of the following detailed description and with reference to two working examples.
DETAILED DESCRIPTION OF THE INVENTION
The present invention is based on the provision of an insulating Al
2
O
3
/TiO
2
structure comprising alternating thin aluminium oxide and titanium oxide sheets grown layer by layer. According to a particularly preferred embodiment, the insulating film is grown from the corresponding volatile precursors by the Atomic Layer Epitaxy method, that is, using a pulsed infeed of the anion and cation initial reactants in an alternating sequence so as to prevent the surface of the substrate located in the reaction chamber from becoming simultaneously exposed to both initial reactants. The thin film growth process comprises a step in which an aluminium oxide layer is grown followed by a step of growing titanium oxide, both steps being repeated in an alternating sequence until the desired film thickness is obtained.
Any suitable volatile precursors of aluminium oxide and titanium oxide can be used including inorganic and organic aluminium and titanium compounds. Particularly useful compounds are represented by aluminium and titanium halogenoids, such as aluminium and titanium chlorides.
The aluminium chloride and titanium chloride layers are converted to the corresponding aluminium and titanium oxides, respectively, by using water
Pitkänen Tuomas
Törnqvist Runar
Berck Ken A
McDermott & Will & Emery
Patel Vip
Planar Systems Inc.
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