Method of producing thin film electroluminescent structures

Method of producing a thin film electroluminescent device by sputtering a first transparent electrode of indium tin oxide or tin oxide onto a glass substrate, sputtering a layer of insulating material, for example barium tantalate, over the transparent electrode, and then forming a phosphor layer of zinc sulfide with manganese as an activator on the layer of insulating material. To form the phosphor layer electrical energy is applied to a target containing elemental zinc in an atmosphere containing hydrogen sulfide and argon to cause sputtering therefrom. Elemental zinc reacts with the hydrogen sulfide to deposit a layer of zinc sulfide over the layer of insulating material. The manganese is cosputtered either from a separate target or from a single target incorporating both zinc and manganese.

BACKGROUND OF THE INVENTION 
This invention relates to thin film electroluminescent devices. More 
particularly, it is concerned with methods of depositing the phosphor 
layers of thin film electroluminescent devices by sputtering. 
Thin film electroluminescent devices are employed for various forms of 
displays. Typically the devices employ a transparent substrate having on 
one surface a very thin conductive electrode which is substantially 
transparent. This first electrode is covered with an insulating layer. A 
layer of a suitable phosphor material overlies the insulating layer. The 
phosphor layer is covered with another insulating layer, and a second 
conductive electrode of an appropriate pattern is formed on the second 
insulating layer. Under operating conditions a voltage is applied between 
the two electrodes causing the portion of the phosphor layer between the 
electrodes to luminesce, thus providing a visible pattern when viewed 
through the transparent substrate. 
Various methods have been used to deposit the layers constituting thin film 
electroluminescent devices including thermal evaporation, electron beam 
evaporation, chemical vapor deposition, and sputtering. From the point of 
view of an ecomonical manufacturing process, it is desirable that all the 
layers be deposited by sputtering. There is no problem as to the 
conductive and insulating layers since sputter deposition techniques for 
the materials typically employed for these layers are well known. 
Heretofore, however, it has been difficult to obtain phosphor layers of 
satisfactory characteristics by employing sputtering techniques. In order 
to obtain satisfactory phosphor layers they have usually been formed by 
employing evaporation methods, which are less easily adapted to 
large-scale production than is sputtering and, therefore, are more 
expensive. 
Typically the phosphor layer is a host of zinc sulfide (ZnS) containing an 
activator, frequently manganese (Mn). In forming zinc sulfide phosphor 
layers by sputtering, the conventional technique includes applying rf 
energy to a target of zinc sulfide enclosed in a chamber containing a 
sputtering atmosphere of argon and a small quantity of hydrogen sulfide. 
One of the disadvantages of this process in producing phosphor layers in 
electroluminescent devices is that during the target erosion process very 
small particles of zinc sulfide tend to form. Many of these particles 
impinge on the substrate and become incorporated into the growing film 
where they represent discontinuities in both the geometric shape and 
crystal structure of the otherwise highly oriented phosphor film. In 
addition after exposure of a zinc sulfide target to air, a long period of 
presputtering is necessary in order to return it to a safisfactory 
condition for use as a sputtering target. 
SUMMARY OF THE INVENTION 
Thin film electroluminescent structures having uniform, high-quality zinc 
sulfide phosphor films are produced by the sputtering method in accordance 
with the present invention. The method comprises providing a substrate of 
transparent material having a transparent film of conductive material 
adherent thereto and a coating of insulating material overlying the 
transparent film. The substrate and a target containing elemental zinc are 
placed in a chamber enclosing an atomsphere containing hydrogen sulfide. 
Electrical energy is applied to the target to cause sputtering therefrom 
whereby elemental zinc reacts with the hydrogen sulfide to deposit a layer 
of zinc sulfide over the coating of insulating material.

For a better understanding of the present invention, together with other 
and further objects, advantages, and capabilities thereof, reference is 
made to the following disclosure and appended claims in connection with 
the above-described drawing. 
DETAILED DESCRIPTION OF THE INVENTION 
The sole FIGURE of the drawing illustrates a fragment of a thin film 
electroluminescent device. The device includes a substrate 10 which is 
transparent and typically is of glass. A thin transparent conductive 
electrode 11 which typically is of indium tin oxide or tin oxide is formed 
on the surface of the glass substrate 10. The conductive electrode 11 is 
usually of a particular predetermined pattern depending on the display. 
The electrode 11 is covered by a layer of insulating material 12 which may 
be silicon nitride, barium tantalate, or other suitable material. The thin 
film electroluminescent phosphor material 13 is deposited on the 
insulating layer 12. Typically the phosphor film 13 consists of a host 
material such as zinc sulfide (ZnS) and an activator such as manganese. A 
second insulating layer 14, which may be of the same material as the first 
layer 12 or of a different material, is deposited over the phosphor film 
13. A second electrode 15, usually of aluminum, is formed on the surface 
of the insulating layer 14 in a predetermined pattern. As indicated 
symbolically by the leads 21 and 22, electrical connections are applied to 
the electrodes 11 and 15, respectively. A voltage applied across the 
electrodes causes intervening phosphor material to electroluminesce, thus 
producing a visible display to an observer looking through the glass 
substrate 10. 
The electrode 11 may be deposited onto the glass substrate 10 by employing 
known sputtering techniques. Similarly, the layer of insulating material 
12 may be deposited on the first electrode 11 by sputtering. After the 
phosphor layer 13 is deposited, either in accordance with prior art 
practices or in accordance with the present invention as will be described 
hereinbelow, the second layer of insulating material 14 is deposited by 
sputtering. The second electrode 15 is usually deposited by employing 
known evaporation techniques. 
In the fabrication of thin film electroluminescent devices in accordance 
with the present invention, zinc sulfide is deposited by a sputter 
reaction process employing a metallic target of elemental zinc in an 
atmosphere containing hydrogen sulfide (H.sub.2 S) and an inert gas. The 
procedure is carried out in conventional sputtering apparatus. Under 
suitable values of substrate temperature and hydrogen sulfide pressure, 
deposits of stoichiometric zinc sulfide can be obtained. More 
particularly, satisfactory results are obtained when the substrate 
temperature is maintained in the range of about 100.degree. to about 
350.degree. C. and the concentration of hydrogen sulfide in the sputtering 
gas is in the range of about 5% to about 20% by volume with the remainder 
being argon. 
In order to provide a useful electroluminescent film the zinc sulfide host 
is activated, or doped, with a suitable activator, typically manganese. 
Activated films may be formed by cosputtering manganese from a separate 
target. In accordance with the teachings in U.S. Pat. No. 4,279,726 to 
Baird and McDonough the substrate may be rotated so as to pass 
sequentially under the zinc and manganese targets. The size of the 
aperture through which the manganese is sputtered and the rf energy 
applied to the two targets are such as to produce a zinc sulfide phosphor 
film with a manganese content in the range of about 0.5 to 1.0 weight 
percent. The manganese may be incorporated with the zinc in a single 
target, either by being dispersed throughout the target or by being 
alloyed with the zinc. The relative amounts of zinc and manganese are 
chosen so that upon sputtering, the deposited phosphor film will have the 
desired proportion of manganese. Activators other than manganese may be 
employed, in particular various of the rare earth elements. Rare earth 
activators may be also employed with a coactivator. 
Devices of the structure illustrated in the drawing were produced by 
depositing indium tin oxide on a glass substrate 10 in a desired pattern 
to form the first electrodes 11 of a thickness of approximately 1000 to 
2000 angstroms. An insulating layer 12 of barium tantalate of 2000 to 4000 
angstroms thick was deposited by sputtering from a target of suitable 
source material in a suitable sputtering gas atmosphere. A zinc sulfide 
electroluminescent layer 13 was formed by cosputtering from one target 
consisting essentially of elemental zinc and another target of manganese 
while rotating the assemblage of the substrate 10 and layers 11 and 12 as 
described hereinabove. Particularly good results were obtained when the 
substrate temperature was maintained at approximately 200.degree. to 
250.degree. C. and the sputtering gas was approximately 10% hydrogen 
sulfide by volume with the remainder being argon. The total gas pressure 
was maintained at approximately 10 microns. The rf power density applied 
to the elemental zinc target was 8 W/in.sup.2. The resulting phosphor film 
13 containing from 0.5 to 1.0 weight percent manganese was between 4000 to 
6000 angstroms thick. The phosphor film 13 was then covered with a second 
insulating layer 14 of barium tantalate 2000 to 4000 angstroms thick which 
was also formed by sputtering. The final electrode 15 was formed by 
evaporating aluminum in the desired pattern. The aluminum electrode 15 was 
between 1000 to 2000 angstroms thick. 
In thin film electroluminescent structures produced in accordance with the 
present invention as described hereinabove, uniform films of 
stoichiometric zinc sulfide were obtained. The particulate matter 
characteristically imbedded in zinc sulfide films sputtered from zinc 
sulfide targets was found to be virtually eliminated. 
An important characteristic of electroluminescent devices is the charge 
density which can be sustained without electrical breakdown. Typically, 
electroluminescent devices are operated at a charge density of 2.5 
.mu.C/cm.sup.2 or less. In many types of prior art devices electrical 
arcing tends to occur in localized regions when the charge density reaches 
1.0 to 1.5 .mu.C/cm.sup.2 as the applied voltage is raised. Normally this 
arcing is "self-healing" in that the aluminum electrode is burned off in 
the immediate vicinity of the arc, and the device returns to normal 
stabilized operation. When the level of charge density is raised to the 
2.5 to 3.5 .mu.C/cm.sup.2 range, however, localized arcing becomes great 
enough to cause the device to burn out without reaching stability. 
A high percentage of devices fabricated in accordance with the present 
invention having individual segments of about 0.05 cm.sup.2 in area were 
brought directly to charge densities of about 4 .mu.C/cm.sup.2 without 
suffering any localized arcing. Also, it was possible to achieve stability 
at charge densities above 5 .mu.C/cm.sup.2 with very few "self-healing" 
breakdown events. In many devices catastrophic device burnout did not 
occur until charge densities of 7 to 8 .mu.C/cm.sup.2 were reached. In 
addition, as the operating voltages of devices fabricated in accordance 
with the invention were raised, lighting was observed to take place in a 
fairly uniform manner. High brightness was obtained without the presence 
of scattered pinpoints of light such as would occur in prior art devices 
indicating discontinuities in the phosphor layer. 
While there has been shown and described what is considered a preferred 
embodiment of the present invention, it will be obvious to those skilled 
in the art that various changes and modifications may be made therein 
without departing from the invention as defined by the appended claims.