There is a transformation brewing in the world of flat-panel displays driven by a new solid-state device called organic light-emitting diode (OLED). Because these devices produce light, rather than reflect or transmit it, they have the potential to produce brighter displays with higher contrast ratios, more color saturation, a 180° viewing angle, and switching speeds measured in microseconds (compared to milliseconds for LCDs). They are ideal for full-motion video applications such as television, where they will not suffer from the image-ghosting problems of LCD TVs trying to handle rapidly moving images.
Because OLED materials are extremely thin — and some are chemically reactive and oxidize immediately on exposure to water or oxygen, creating black spots that ruin the display — they can be 10,000× more sensitive to moisture and oxygen than LCDs. To protect them, display makers currently use glass as the display substrate (the same as LCDs) and glue a glass lid on top, with a desiccant powder inside the display to absorb moisture that comes through the glue line. This design works but is awkward and costly.
A solid-state solution
Currently, a number of FPD makers are evaluating a thin-film solution that offers moisture and oxygen permeability approximately equal to a sheet of glass. It comprises alternating layers of polymer and ceramic films applied in vacuum. The total thickness of the coating is only ~3µm, and it can be applied directly on top of an OLED display, eliminating mechanical packaging components.
A liquid precursor is flash-evaporated to a gas, which then flows into a vacuum chamber where it condenses back to a liquid and onto a substrate. It is not a traditional vacuum process such as evaporation, sputtering, or chemical vapor deposition. All these are gas-to-solid deposition processes in which atoms or molecules hit a substrate in a line-of-sight path and are converted back to the solid state. By their very nature, these deposition processes create conformal layers that have the same topography and surface roughness as the underlying substrate.
In contrast, the polymer layer formed in this new vacuum process is actually condensation of gas to liquid. The precursor gas molecules travel to the substrate and condense on all its surfaces, thereby encapsulating and planarizing the entire structure. Figure 1 shows atomic force micrographs, demonstrating how the coating covers all the imperfections and provides a flat surface.
 Figure 1. Atomic force micrographs of a) PET without coating, and b) PET with coating. |
In addition, because it is a liquid, the flat surface of the monomer is atomically smooth. The substrate next moves to an ultraviolet light source, which polymerizes the liquid to create a solid polymer film, still with an atomically smooth top surface. This provides an ideal surface on which to deposit a barrier film. Next, a ceramic film, ~500Å thick, is deposited on top of the polymer layer. Because the surface is so smooth, the ceramic film has very few defects and is therefore an almost perfect moisture barrier. An OLED display, however, requires an even better barrier, so the process is repeated, creating a stack of multiple polymer and ceramic layers in which each ceramic film is a near-perfect moisture barrier. This combination of ceramic and polymer layers, with a total thickness of ~3µm, creates a moisture barrier with a water permeability in the range of 10-6gm of water/m2/day. This is the water impermeability required by an OLED display. Figure 2 is a SEM cross-section of the stack.
 Figure 2. SEM cross-section of the stack of multiple polymer and ceramic layers. |
There are two basic types of OLED materials: small-molecule materials developed by a.o. Kodak and Universal Display Corp., and polymer materials developed by Cambridge U. Regardless of the type of OLED material, the manufacturing step immediately before encapsulation is deposition of the cathode layers. These are low work-function metals such as aluminum and calcium and are the most sensitive components in the OLED display in terms of exposure to moisture or oxygen. Since the cathode layers are deposited in a vacuum chamber, the display substrate can remain in vacuum and move directly into the thin-film encapsulation tool.
The application of this multilayer polymer/ceramic encapsulating thin film is challenging, and the following factors have to be considered:
- The organic emissive layers in the OLED display are extremely thin, on the order of nanometers, and have little mechanical strength. Subjecting the OLED layers to shear stresses when the monomer is polymerized and solidifies, which involves about 2% shrinkage, is a concern.
- OLED materials are sensitive to the UV light used to initiate polymerization. Consequently, the formulation of the monomer as well as the UV intensity and duration must be carefully controlled. This is especially critical with top-emission displays, since they have transparent cathodes and the OLED layers would be directly exposed to the UV light.
- A plasma is typically used in depositing the ceramic barrier layers, which can also damage the OLED layers and must be carefully controlled.
- Temperature excursions during UV curing and sputtering must be avoided, as many OLED materials would be damaged by temperatures >100°C.
As with most manufacturing processes for semiconductors or flat-panel displays, applying thin-film encapsulation to an OLED display demands tight control of particles. The thin-film encapsulation tool is connected directly to the OLED vacuum tool, where particulate is already tightly controlled. It is essential to ensure that the organic and inorganic deposition processes used in building the multilayer barrier stack do not create any particulates.
Finally, to qualify the thin-film encapsulation process, encapsulated OLED displays must be subjected to high temperature and humidity — typically 60°C/90%RH for 500 hours — as well as thermal shock testing to ensure that the displays will satisfy the requirements of mobile electronic-device manufacturers (i.e., for cell phones and PDAs).
An enabling technology for OLED TVs
Besides being extremely thin yet impermeable to water, the Barix coating is also transparent to visible light. This means OLED display makers could conceivably avoid having to make a bottom-emission display in which the light path is partially blocked by the TFT silicon transistors on the substrate, thereby reducing the display efficiency and placing a limit on resolution. If the mechanical packaging — metal cans, glass lids, and desiccant — were replaced by transparent thin-film encapsulation, then the display could be designed so that all the light exits the top of the display, significantly boosting efficiency and enabling much higher resolution.
This efficiency increase means more than just saving electrical power. The other limiting factor with OLED displays (aside from protection from moisture) is the lifetime of the emissive materials, especially blue emitters. Unlike LCDs, which are voltage-driven, OLED displays are current-driven. Moreover, the amount of current that flows through the emissive materials has a major effect on lifetime. More efficient top-emitter displays require much less current for a given brightness because they avoid the inefficiencies of bottom-emitters where light is partially blocked, and thus have longer lifetimes. A thin-film moisture barrier that meets OLED display requirements is therefore an enabling technology for OLED TVs.
Conclusion
For many years, flat-panel display makers have dreamed of eliminating glass substrates and building their displays on flexible plastic films instead. This has not been possible due to the water permeability of polymer films. The moisture barrier requirements for OLED displays are especially severe — 10,000× greater than any commercial plastic barrier film. A plastic barrier substrate with moisture permeability low enough for an OLED display can be produced using a vacuum roll-to-roll process combined with the thin-film moisture barrier technology described.
Acknowledgment
Barix is a trademark of Vitex Systems.
Malcolm J. Thompson received his BSc and PhD degrees from Brighton U., and is the CEO of Vitex Systems, 3047 Orchard Parkway, San Jose, CA, 95134; ph 408/519-4430, fax 408/519-4472, e-mail mthompson@.vitexsys.com.
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