Transparent, flexible permeability barrier for organic electroluminescent devices

A barrier for preventing water or oxygen from a source thereof from reaching a device that is sensitive to water or oxygen. The barrier is constructed by depositing a first polymer layer between the device and the source. An inorganic layer is deposited on the first polymer layer of the device by plasma enhanced chemical vapor deposition utilizing an electron cyclotron resonance source ECR-PECVD. A second polymer layer is then deposited on the inorganic layer. The inorganic layer is preferably an oxide or nitride. A second barrier layer having a compound that absorbs oxygen or water can be placed between the inorganic layer and the device to further retard the passage of oxygen or water. The present invention is particularly useful in encapsulating electroluminescent displays.

The present invention relates to electroluminescent devices, and more 
particularly, to an improved method for sealing an organic light emitting 
display to increase the lifetime of the display. 
BACKGROUND OF THE INVENTION 
Organic light emitting devices (OLEDs) are emissive displays consisting of 
a transparent substrate coated with a transparent conducting material, 
such as Indium Tin oxide (ITO), one or more organic layers, and a cathode 
made by evaporating or sputtering a metal of low work function 
characteristics, such as Ca or Mg. The organic layers are chosen so as to 
provide charge injection and transport from both electrodes into the 
electroluminescent organic layer (EL) where the charges recombine, 
emitting light. There may be one or more organic hole transport layers 
(HTL) between the ITO and the EL, as well as one or more electron 
injection and transporting layers (EL) between the cathode and the EL. 
OLEDs hold out the promise of providing inexpensive displays. In principle, 
these devices can be manufactured on flexible substrates and fabricated 
using "roll-to-roll" processing equipment. Inexpensive equipment for such 
fabrication operations such as polymer film coating devices, metal 
evaporators and lithography equipment capable of providing the deposition 
of the various layers are already available. For example, Web coating 
devices for thin polymer films that are a few feet wide can operate at a 
feed rate of hundreds of feed per minute. 
To function over extended periods of time, an OLED must be sealed to 
prevent water and oxygen from reaching the cathode and polymer layers. 
Unfortunately, polymers having sufficiently low permeability to water and 
oxygen are not available. For example, Poly(ethylene terephthalate) or 
PET, which is used as a command substrate for Web processing has a water 
permeability that is so high that devices constructed thereon begin to 
degrade almost immediately due to reaction of water from the air with the 
cathode material. Accordingly, some form of sealant coating must be 
applied to the polymer to achieve the required resistance to water and 
oxygen. In addition, the cathode layer must be sealed on the other side of 
the device to prevent water and oxygen from entering from that side and 
destroying the cathode. 
One coating technique that has shown promise is the Polymer Multilayer 
(PML) technique described in U.S. Pat. Nos. 4,842,893, 4,954,371, and 
5,260,095. In this technique, a coating consisting of a layer of polymer 
and an layer of an aluminum oxide is applied to the flexible substrate to 
seal the substrate. Both the deposition steps can be operated on Web 
processing equipment at very high speeds. While the resistance to water 
and oxygen permeation is improved by three to four orders of magnitude 
relative to uncoated PET films, the resulting films are still sufficiently 
permeable to limit the lifetime of the OLEDs in application requiring 
lifetimes of several years and/or exposure to hot humid environments. 
Using accelerated lifetime test procedures, it can be shown that the 
permeation rate should not exceed about 4.times.10.sup.-7 moles H.sub.2 
O/m.sup.2 day in order to have a storage lifetime of 10 years. The best 
films currently available have permeabilities that are at least a factor 
of 50 too high. It should be noted that applying several polymer bilayers 
does not improve the resistance to water and oxygen sufficiently to 
provide the required increase in resistance. 
Broadly, it is the object of the present invention to provide an improved 
OLED display and method for making the same. 
It is a further object of the present invention to provide a method for 
constructing a PML that has sufficient resistance to water and oxygen 
permeation to provide OLEDs having commercially useful lifetimes. 
These and other objects of the present invention will become apparent to 
those skilled in the art from the following detailed description of the 
invention and the accompanying drawings. 
SUMMARY OF THE INVENTION 
The present invention is a barrier for preventing water or oxygen from 
reaching a device that is sensitive to water or oxygen. The barrier is 
constructed by depositing a first polymer layer between the device and the 
source. An inorganic layer is deposited on the first polymer layer of the 
device by plasma enhanced chemical vapor deposition utilizing an electron 
cyclotron resonance source ECR-PECVD. A second polymer layer is then 
deposited on the inorganic layer. The inorganic layer is preferably an 
oxide or nitride. A second barrier layer having a compound that absorbs 
oxygen or water can be placed between the inorganic layer and the device 
to further retard the passage of oxygen or water. The present invention is 
particularly useful in encapsulating electroluminescent displays.

DETAILED DESCRIPTION OF THE INVENTION 
The manner in which the present invention gains its advantages may be more 
easily understood with reference to FIG. 1 which is a cross-sectional view 
of a portion of an OLED display 100 according to the present invention 
based on the color conversion methodology. Display 100 is constructed on a 
plastic substrate 161 such as the PET substrate material discussed above. 
The light that is converted to the primary colors is generated in an EL 
layer 118. The light in EL layer 118 is generated by connecting row and 
column electrodes to the appropriate power supplies. For the purposes of 
this discussion, the row electrodes 117 will be assumed to be the anode of 
the device, and the column electrodes will be assumed to be the cathode. 
The cross-sectional view shown in FIG. 1 is taken through one row 
electrode. The column electrodes run at right angles to the row 
electrodes. Typical column electrodes are shown at 131-133. Light 
generated at the intersection of a row and column electrode illuminates a 
color conversion strip that lies below the column electrode. The color 
conversion strips corresponding to electrodes 131-133 are shown at 
121-123, respectively. Strip 121 converts the light emitted by layer 118 
to blue light. Similarly, strip 122 converts the emitted light to green, 
and strip 123 converts the emitted light to red. The portion of the 
display shown in FIG. 1 includes the column electrodes for three full 
color pixels shown at 113-115. 
The various organic layers that make up the light emitting and injection 
layers are deposited over the patterned anode electrodes. To simplify the 
drawing, these layers are shown as a single light emitting layer 118; 
however, it is to be understood that layer 118 may be constructed of a 
number of sub-layers that facilitate the injection of holes and electrons 
into an EL layer as described above. Since the fabrication of such a 
multilayer structure is conventional in the art, it will not be discussed 
in detail here. It is sufficient to note that the layers can be deposited 
by spin casting, dye sublimation, web coating, or various "printing" 
techniques depending on the particular material system chosen. 
The cathode lines are constructed from a low work-function material such as 
calcium or magnesium. Shadow masking techniques for depositing such 
electrodes are also well known in the art, and hence, will not be 
discussed in detail here. 
As noted above, the final displays need to be encapsulated to prevent 
oxygen and moisture from penetrating to the cathode electrodes and the 
light emitting layers. Such encapsulation layers are shown at 171 and 172 
in FIG. 1. In the present invention, the encapsulating layers are 
constructed as PML layers analogous to those described above. Each 
encapsulation layer includes two polymer layers and a layer of inorganic 
oxide or nitride sandwiched therebetween. The polymer layers corresponding 
to encapsulation layer 171 are shown at 181 and 183, respectively. The 
oxide or nitride layer corresponding to encapsulation layer 171 is shown 
at 182. Similarly, the polymer and oxide or nitride layers corresponding 
to encapsulation layer 172 are shown at 191, 193, and 192, respectively. 
The polymer layers are deposited by evaporating a monomer for the polymer 
into a coating chamber, which is typically evacuated. The portion of the 
device that is to be coated is maintained at a temperature below the 
boiling point of the monomer by contacting that portion with a cold 
surface, typically a roller that the device is moving over during the 
coating process. The monomer solution condenses on the device forming a 
uniform liquid coat that fills in the various gaps thereby planarizing the 
surface. The monomers are then cross-linked by exposure to a radiation 
source such as a UV lamp. The deposition of the polymer layers is 
discussed in detail in the above-cited patents, which are hereby 
incorporated by reference, and hence, will not be discussed in further 
detail here. It is sufficient to note that the resulting polymer surface 
is exceptionally smooth. 
The inorganic oxide is applied over the polymerized polymer surface. In 
prior art encapsulation systems utilizing the PML technology, the oxide 
was applied by sputtering or evaporation. The extremely smooth polymer 
surface provides a low defect surface for the application of the oxide. 
Accordingly, the oxide has relatively few pinholes through which oxygen or 
water can travel. However, the oxide layer still passes sufficient oxygen 
and/or water to limit the device lifetime. The present invention overcomes 
this limitation. 
The permeability of the oxide layer is determined both by the density of 
pinholes in the layer and density of the oxide material. The PML 
technology addresses the pinhole problem; however, the deposition 
techniques utilized in the prior art systems do not provide a sufficiently 
dense oxide layer to limit permeability to the desired levels. The present 
invention is based on the observation that oxide or nitride layers 
deposited by plasma enhanced chemical vapor deposition utilizing a high 
density plasma, particularly an electron cyclotron resonance source 
(ECR-PECVD) have significantly higher densities than those deposited by 
the methods taught in the prior art, while allowing deposition under 
conditions that do not damage the underlying polymer layers. 
High-density plasmas are characterized as having a very small sheath 
voltage, on the order of 5 times the electron temperature in eV, at 
surfaces containing the plasma. This is in contrast with low density 
plasmas which have capacitive coupling and high sheath voltages at walls. 
In capacitive plasmas the power into the plasma is coupled to the 
potential of ions striking the walls. In high-density plasmas, the 
potential of ions striking the walls is inherently very low and can be 
controlled by adding capacitively coupled power at the substrate. Hence, a 
high-density plasma provides a high flux of low energy ions along with a 
high flux of reactive species that are generated at the surface to be 
coated. This enables the deposition of usable dielectrics at temperatures 
compatible with polymers. Dielectrics such as silicon nitride, silicon 
dioxide, aluminum oxide, silicon carbide, silicon oxynitride, and such can 
be deposited utilizing this technique. 
A number of high-density plasma systems are available. For example, such 
systems may be purchased from PlasmaQuest, Plasma-Therm, Surface 
Technology Systems, Trikon, Lam Research, Applied Materials, Tegal and 
Novellus. 
ECR is not necessary to create a high-density plasma and other schemes have 
been commercialized such as Inductively Coupled Plasma, Helical Resonator, 
and Helicon plasma. As an example for illustration only, we have found 
that an Oxford Instruments ECR system, using silane, argon and nitrogen as 
source gases at a pressure of between 5 and 10 mTorr, with 700 Watts of 
2.45 GHz microwave power and a magnetic field of about 875 Gauss, gave 
excellent results as described below. These operating parameters may be 
varied considerably as will be evident to those skilled in the art, to 
optimize conditions for particular substrates. 
To demonstrate the present invention, a silicon nitride film was deposited 
by this technique on a thin calcium film (about 200 angstroms, prepared by 
thermal evaporation of calcium) on glass. The temperature of the film was 
maintained below 80.degree. C. by a cooling stage. Upon exposure to air, 
the metal adjacent to pinholes became transparent, and the transparent 
diameter grew slowly larger with time. The remaining metal, however, was 
unchanged in appearance in the microscope after 20 days. Given that a 
reaction of even 1% of the calcium would be visible (because of the 
surface disturbance), this observation gives an upper limit to the 
permeation rate of about 8.times.10.sup.-7 moles H.sub.2 O/m.sup.2 day. 
The etch rate of the film in a 10% (by volume) aqueous HF solution, which 
is a good measure of its morphological quality, was 120 nm/min, which by 
comparison with published data (Y. Tessier, et al., Mat. Res. Soc. Symp. 
Proc. Vol. 86, 1986, p. 183) is indicative of a water permeability of 
around 1.times.10.sup.-8 mol/m.sup.2 day, 40 times better than the 
requirement for organic LEDs. 
A further important advantage of the present invention is that the stress 
of the deposited inorganic film can be varied, depending on the parameters 
such as the microwave power and gas pressure. The reduced stress reduces 
the distortions of the patterned elements and prevents delamination or 
distortion as the substrate is rolled up during roll-to-roll processing. 
The example film had a stress of 2.5.times.10.sup.-9 dynes/cm.sup.2, which 
is quite low. 
A further advantage of the present invention is an improved deposition rate 
over the prior art methods discussed above. With the method of the present 
invention, rates of more than 1 micron per minute can be achieved. Since 
the deposition rate determines the maximum rate at which the roll-to-roll 
processing equipment can operate, the present invention provides increased 
throughput. 
In the preferred embodiment of the present invention, the polymer layers 
are constructed from an acrylic monomer, such as methyl methacrylate, 
ethylene glycol diacrylate, or other acrylates, diacrylates, and 
triacrylates or methacrylates having a thickness of about 1 micron. The 
oxide layer is preferably silicon nitride having a thickness of 350 .ANG.. 
However, embodiments utilizing other oxides or nitrides may also be 
constructed. For example, layers 182 and 192 can be constructed from 
silicon or aluminum oxides or aluminum nitride. The second polymer layer 
is used to protect the nitride layer from subsequent damage during use and 
processing. This layer is also preferably constructed from an acrylic 
monomer having a thickness of 0.25 microns or greater. However, in both 
polymer layers, other monomers, such as carbonates, fluorinated alkenes, 
or other liquid monomers that can be cured to an insoluble film, can be 
utilized 
While the above-described embodiments of the present invention utilize an 
encapsulation layer that is on the opposite side of the substrate film 
from the anode electrodes, embodiments in which the substrate is coated 
with a PML film and the anode applied to the polymer layer may also be 
constructed. In general, the smooth surface provided by the polymer layer 
is superior to that provided by a PET substrate. Hence, the resultant 
anode electrodes have fewer defects than those obtained by application of 
the ITO layer directly to the PET. In addition, the PET coated substrate 
can be prepared separately and stored, thereby relieving the manufacturer 
of the OLED device of the need to perform the first encapsulation 
operation. 
The barrier system of the present invention may be further enhanced by 
including a layer that absorbs oxygen and/or water. Such a "getter" layer 
can be constructed by providing a layer of material that reacts with 
oxygen or water that penetrates the layer and thereby removes the oxygen 
or water before it reaches the display device. In the simplest embodiment, 
a small amount of a getter compound is included in the oxide or nitride 
layer itself For example, a low concentration of metallic lithium can be 
added to an oxide or nitride layer that is formed by co-evaporation or 
co-sputtering from two sources. 
The concentration must be sufficiently low to maintain the transparency of 
the barrier if the layer is on the light emitting side of the device. The 
lithium will react with oxygen and/or water until the metallic lithium is 
expended, and the permeability of the layer will be lower than it was 
initially, because the free volume is now occupied by the chemically bound 
oxygen atoms. Hence, the device will be further protected for some period 
of time from oxygen or water that would have otherwise penetrated the 
barrier. 
Alternatively, the getter layer can be placed between the barrier and the 
device as a separate layer as shown in FIG. 2 at 200. In this case, a 
separate layer of getter material 201 is placed between the PML barrier 
layer 202 and the device 203. An additional polymer layer 204 may be 
utilized to separate the getter layer from the device. A similar structure 
is provided on the other side of the device to prevent oxygen and/or water 
from penetrating from that side as well. The PML smoothing layer improves 
the quality of the getter layer compared to those described in the prior 
art, for example in the U.S. Pat. Nos. 5,047,687 and 5,059,861. The getter 
material may be any metal that reacts rapidly with water, such as calcium, 
magnesium, or any alkali metal. Because the metals are protected by the 
PML-based water/oxygen barrier, highly reactive metals that are otherwise 
hard to handle may be used. 
The barrier system of the present invention has been described in terms of 
preventing water or oxygen from reaching the active layers of an OLED. 
However, it will be obvious to those skilled in the art from the preceding 
discussion that the method of the present invention may be used to provide 
an oxygen or water barrier for other types of devices or films. 
Various modifications to the present invention will become apparent to 
those skilled in the art from the foregoing description and accompanying 
drawings. Accordingly, the present invention is to be limited solely by 
the scope of the following claims.