Note: Descriptions are shown in the official language in which they were submitted.
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D-22,048
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A THIN ~IL~ PJL~CTROLUMINESC~NT DISPLAY DEVIC~
BACKGROU~D OF TH~ INVENTION
The present invention relates in general to a ~hin
film elec-troluminescen~ display device and is concerned,
more par~icularly, with an improved dark field material
for s~lch a thin film elec~roluminescent display device.
~ lec~roluminescen~ devices ~enerally comprise a
phosphor layer disposed between two elec~rode layers
with olle of the electrodes being transparen~ so as to
permit viewability of the phosphor layer. It is known
to provid~ a dark field layer behind the phosphor layer
in order to improve the con~rast ratio of the device
when u~ing a seymented back electrode layer; that is to
say, to provide visibility of the phosphor layer
overlying the back electrod~ segments even under ambient
condi~ions of high brightness. See. U.S. Patent
3~560,7~4 for an example of a dark field layer, the
material of which may comprise ~rsenic sulphide, arseniG
selenide, arsenic sulfoselenide or mixtures thereof.
However, these arsenic co~pounds ei~her do not provide a
satisfactory dark color or ~hey change color during use.
Perhaps ~he most common dark field material presently
being used is cadmium telluride (CdTe). Althou~h the
CdTe layer provides for enhancement in contrast between
~he displayed information and the background, one of the
problems associated with the CdTe composition is that it
i5 toxic and the material does not meet sa~ety
specifica~ions for commercial produc~s as reguired by
OSHA (Occupational Safe~y and Heal~h Act~.
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D-22,048
Accordi~g to one prior solution ~o ~his toxicity
proble~5 an electroluminescent device has a dark field
layer comprising a cermet of chromium oxide - chromiu~
(Cr203/Cr). Although overcoming the toxici~y
problem,
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this cermet comprises a combination Df ~ metal (Cr~ and an o~de
~Cr203) of the same base metal9 thereby renderins the dar~ field
composition d1fficult, if not impossible, for an61ysis of the
constituent proportions. Such analysi~ is important to enable
5 precise control of the constituent proportion for providin~ optimum
results.
Accordin~ly, it is an object of the present invention to provide
an improved electroluminescent display device and in particular an
improved dark field material for such a device.
Another object of the present invention is to provide an
improved dark field in accordanre with the precedin~ object and
which is characterized by an improved contrast ratio of the device.
Still another object of the present invention is to provide a
dark field material in accordance with the precedin~ objects and
which is non-to~ic and meets the safety specifications for
commercial products required by OSHA.
A further object of the present invention is to provide an
improved dark field layer in a thin film electroluminescent display
device in which for at least some applications, only a single
transparent dielectric layer of the device is employed in comparison
with the typical first and second transparent dielectric layers used
in the past in electroluminescent thin film display de~ices.
Still a furtber object of the present invention is to provide an
improved dark field material for a thin film electroluminescent
display device in which the dark field layer is formed of
constituents which are readily analyzable, and thus precisely
controllable, to provide enhanced fle~ibility in controllin~
parameters of the dark field layer such as contrast ratio.
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According to one aspect of the present invention there
is provided an electroluminescent display device comprising a
transparent electrode layer, a segmented electrode layer,
an electroluminescent phosphor layer disposed between said
electrode layers, and a dark field layer of a composition of
a dielectric material with a noble metal, said dark field layer
being interposed between said electroluminescent phosphor layer
and said segmented electrode layer, wherein there is only a
single transparent dielectric layer adjacent the electrolumines-
cent phosphor layer.
BRIEF DESCRIPTION OF THE DRAWINGS
Numerous other objects, features and advantages of theinvention should now become apparent upon a reading of -the
following detailed description taken in conjunction with the
accompanying drawings, in which:
FIG. 1 is a schematic cross-sectional view showing the
multiple layers of a thin film elec-troluminescent display
device including the dark field layer oE -this invention; and
FIG. 2 is a schematic cross-sectional view showing an
alternative construction of the thin film electroluminescent
display device showing a single transparent dielectric layer
rather than the two dielec-tric layers depicted in FIG. 1.
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DESCRIP~ION OF PREFERRED E~80DI~ENT
In accordance with the present invention, the dark field
msterial for a thin film ele~trolum;nescent display devic~ is formed
by a composition of 8 dielectric material with a noble metal. The
dark field layer serves the basic purpose oi` enhancin~ the contrcst
between the displayed information which is usually in segment form
and the back~round. In order to eliminste the prior art problem
associated with CdTe d~rk field layers, which are toxic, and yet
provide suitable analyzability o~ the dark field composition, it has
been found in accordance with the present invention that a
composition of 7 for example, magnesium oxide and gold which are
co-evaporated, preferably by an electron beam technique, provide a
dark field material that is non-toxic, is readily analyzabl~e ~nd
meets the safety specifications for commerclal products. A layer of
such material has not previously be~n employed at all in the
construction oP electroluminescent display devices, although, a
~O/Au film has been previously evaluated as a solar absorbin~
material for solar pane-ls. In this re~ard, see U.S. Patent
4,312,915; also see the article by Fan and Zavracky, Applied Physics
Letters, Volume 29, No. 8, 15 October, 1976, pa~e 478-480. Also see
the article by Berthier and Lafait in Thin Solid Films 89 (1982~
213~220 entitled "~ptical Properties of Au-M~O Cermet Thin Films:
Percolation Threshold and Grain Size Effect". The latter article is
concerned primarily with the method of deposition and associated
optical properties.
In addition to the advantage of non-to~icity of the composition
of this inventlon, the lsyer has also been found to unexpectedly
provide contrast enhancement.
Uith reference to the drawin~, it is noted that in FIG. 1 tbere
is shown a ~ersion of an electroluminescent display device
lncorporatin~ the dark field of this invention. In FIG. 2, one of
the two transparent dielectric layers shown in FIG. 1 has been
removed because, in accordance with the present invention~ the
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D--22 ,048
improved dar~ field layer also functions n~ a substltute for one of
the dielectric layerg. In other words the dielectric/noble metal
composition serves both as the dark field and as the second
dielectric.
In FIGS. 1 and Z, li~e reference charscters are used to identify
li~e layers of each embodiment disclosed. Thus, there is showm a
glass substrate 10 on which are formed a number of multiple
thin-film layers, which may be enclosed by a glass seal 11. These
layers include a transpare~t electrode 12, a first transparent
dielectric layer 14, ar electroluminescent phosphor lay~r 16, a
second transparent dielectric layer 18, a dark field layer 20, and a
back se~mented electrode 22. In FIGS. 1 and 2 the transparent
dielectric layers may be of yttria, and the electroluminescent
phosphor layer may be of, for e~ample, zinc sulphide. In the
embodiment of FIG. 1, the second transparent dielectric layer 18 is
shown, but it is noted that in the embodiment of FIG. 2, this layer
i5 not present., The dark field layer 20 in FIG. 2 ;nstead serves
both as the dark field and as the second dielectric layer.
The composition of the dark field layer 20, which in its
broadest sense comprises a dielectric material, preferably a
ceramic, and a noble metal, preferably ~old, may be deposited by
co-evaporation using standard deposition techniques. In accordance
with one technigue, co-evaporation is used with e-beam equipment.
The evaporation may take place in one chamber of a two-chamber
system. The two chamber system has two e-beam guns, each with its
own power supply. In the preferred version, ma~nesium oxide may be
in pellet form and loaded into one crucible, and gold is disposed in
the secoDd crucible. The deposition may be measured by means oP
conventional crystal monitors. One crystal monitor is placed over
each crucible bein~ disposed as close as possible to the position
where the substrate is. The co-evaporation technigue usin~ separate
crucibles is carried out iD a vacuum of preferably better tha~ 1 x
torr. The volume percentage of ~old is varied with the gold
concentration preferably in the range of 6%-10~ by volume.
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The percentage of gold in the composition is
instrumental in controlling ~he resistivi~y of the
cerme~.
In one test that was carried out~ the dark field
layer had a thickness of 0,5 micron. The preerred film
thickness is in the range of 5000-9000 Angstroms. The
la~eral resistance between back electrode seg~en~s i8 on
the order of 10 megohms while the perpendicular
resis~ance across the f;lm thickness is on the order of
lK ohm or less. A contrast ratio of 2:1 is mea~ured at
an ambient light lsvel of 2500 foot-candles with the
back electrode segments at 160 ~ol~s and 60
foo~-candles, Wi~h those parameters, display devices
have baen operdted successfully up to 500 hours of
operating time,
With regard to measurements of contrast be~ween the
displayed informa~ion and the background, such
measursments have been taken by shining a Sylvania
Sun-Gun lamp at ~he lighted and unlighted display
segments. The Sun-Gun lamp was set at an ou~put of 3500
foot-candles. In two differen~ respective devices ~hat
were ts~te~, the contrast ratio measured wa~ 4.2 and
5.3, respectively.
In accordance with another technigue for forming the
dark field layer~ sputkering may be used in a reac~ive
a~mosphere of say argon and oxygen in a ratio of
70%-30%, respectively.
One of the primdry advantages of the composition
MgO/Au is that the material itself as well as the
process forming it, is non-toxic, ~lso, ~he admixed
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~etal (Au) and the metal of the ~etal oxide (~y) are ~WQ
different ma~erials and thus the ratio be~een these
constituents is readily analyzable and, ~hus, provides
f~r an added degree of control over such parameters o~
the dark field layer as electrical conductivi~y and
optical absorption.
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Reference has been made to the preferred layer construction of
magnesium oxide and ~old. Howsver, it is understood that in
accordance with other embodiments of the inYention the composition
may comprise other noble metals in plac of the gold such as
platinum or silver. The dielectric portion of the composition may
be a ceramic. This can be a metal o~ide or a metal nitride (such as
aluminum nitride) or can even be a semiconductor such as silicon
dio~ide or ger~anium dio%ide. Ths noble metal portion of the
composition is in the form of a relatively stable metal thus not
tendin~ to react with the metnllic in the ceramic portion of the
composition. The noble metal, such a gold does not readily oxidize
if it is mixed with the magnesium ox;de.
Havin~ now described fl limited number of embodiments of the
p~e~e~t lnvention, it should now be apparent to those skilled in the
art that numerous other embodiments are contemplated as falling
within the scope of this inYention as defined by the appended
claims, for e~mple, the dark field layer may be deposited by
techniques other than co-e~aporation or electron beam evaporation,
such as by sputtering.
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" SUPPLEMENTARY DISCLOSURE
In the Principal Disclosure there is shown an electro-
luminescent display device comprising a transparent electrode
layer, an electroluminescent phosphor layer, and a dark field
layer. This embodiment has since been modified to provide
an improved dark field layer in a thin film electroluminescent
display device in which, for at least some applications, only a
single transparent dielectric layer of the device is utilized and
to provide an improved dark field material in which the dark
field layer is formed of constituents which are readily analyzable,
and thus precisely controllable, to provide enhanced flexibility
in controlling parameters of the dark field layer such as spectral
transmission.
According to another aspect there is provided an electro-
luminescent display device comprising a glass substrate, a -trans-
parent electrode layer, a dielectric layer, an electroluminescent
phosphor layer over said dielectric layer, a dark field layer
disposed above said phosphor layer and a segmented electrode layer,
said dark field layer of a non-toxic composition of a dielectric
materlal with a noble metal having a spec-tral transmission of less
than about 20% in the range of about 400 nanometers to about 800
nanometers (nm).
In the drawings:
FIG. 3 is a graph illustrating the transmission curves
of three cermet samples having varying concentrations of gold as
the noble metal.
Referring to the figures, initially, one of the experi-
ments that was carried out with the MgO-Au cermet involved using
25~ by volume of gold in the dark field layer. The films were
formed to be totally opaque but were highly conductive, therefore
all subsequent films were made with less gold, ranging between
6%-14% (by volume) of. gold. Figure 3 illustrates the transmiss-
ion curves of three cermets with varying amounts of gold as the
noble metal. In this particular embodiment, gold is used as the
noble metal but it is possible to substitute platinum or silver
as the noble metal and to develop transmission curves in wave-
lengths having a range of about 400 nanometers (nm) to about 800
nm (the visible range). Curve A illustrates a cermet sample
with 93.5% MgO and 6.5% Au for the range from 400 nm-800 nm and
a thickness of about 4500 Angstroms. Curve C illustrates a
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cermet sample with 86% MgO and 14% Au for the same wavelength
range and a film thickness of about A800 Angstroms. The sample
of curve A exhibited good dielectric properties, but the film
was too transparent; the sample of Curve C was sufficiently opaque,
but did not satisfy the electrical requirements of lateral resist-
ance of greater than or equal to 10 megaohms (M n ).
The sample of curve B, on the other hand, which had
92% MgO and 8% Au and a film thickness of about 9000 Angstroms,
satisfied the optical as well as the electrical requirements by
having a spectral transmission below 20%, a lateral resistance
in the order of 10 megaohms and a perpendicular resistance below
lK ohm. Spectral transmission values of above 20% (as in Curve A)
would most likely prove to be transparent while values too far
below 20% could be sufficiently opaque but may prove to be too
conductive. The present film was used in a display device with
a second dielectric layer as well without an additional dielectric
layer where the dark field acts itself as a second dielectric.
The preferred film thickness of the sample of Curve C
was between about 5000 and about 9000 Angstroms for the cerme-t
utilizing gold as the noble metal and a spectral transmission
percent of about 20% for a wavelength that approached 800 nm (see
Figure 3). It is evident from this embodiment that for other
noble metal cermets, the volume percent of metal and the film
thickness can be varied to arrive at the preferred optical
(% spectral transmission) and electrical (perpendicular and lat-
eral resistances) properties.
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