"Multilayer Barrier Coatings as Encapsulation and as Substrates for Flexible Displays and Flexible Electronics "
R. J. Visser
Chief Technology Officer, Vitex Systems
Flexible displays will need barrier coatings against water and oxygen in order to protect the display and its backplane from degradation. Organic Light Emitting Diodes (OLED) have an extremely high sensitivity against water, needing a barrier film with a Water Vapour Transmission (WVTR) of as low as ~10-6 gr/m2/day, but also LCD displays and Electrophoretic displays need more protection then a normal plastic film with a typical WVTR of 1~10 gr/m2/day can provide. For making a flexible display, one not only needs a barrier substrate (even a flexible metal foil can be seen as such), but one also needs to protect the display from the other side. This can be done by thin film encapsulation or by sandwiching the display between two barrier films.
But although thin film barrier coatings on plastic and thin film encapsulation are highly desirable, it has not been so easy to achieve that goal in a technically and economically feasible way. The requirements to the layers of being transparent, totally pinhole and crack free over very large (>1 m2 ) surface areas, low stress and high robustness while being deposited at low temperatures well below 80 C, have proven to be very difficult to meet. Early attempts to solve this problem with single layer oxides or nitrides, while obtaining some success on small areas, basically failed because of the presence of particles, crack and defects in the layer and residual stress. Vitex has proposed a multilayer of organic and inorganic layers, Barix (TM) , to address and solve these problems. The multilayer consists of thicker polymer layers alternated by thin layers of oxide or nitride. The polymer layers are being deposited in vacuum as a thin liquid film of an acrylate monomer which is polymerized with UV light. These layers fulfill the following functions: because of their initial liquid state they planarise the substrate and because of the flat surface of these films, provide the almost ideal surface to grow a defect free oxide. The polymer layer furthermore covers particles, decouples defects in the oxide layers so that they are not aligned and function as a stress release layer. The thin films of oxide serve as the barrier layers to oxygen and water.
Figure 1 - SEM Cross section of a typical Barix multilayer barrier coating. Oxide layers typically are between 30-100 nm and polymer layers 0.25 to 4 micrometersp>
The layers are all deposited in vacuum. The organic layers are applied as follows: a mixture of photosensitive acrylate monomers is vaporized, condensed on the substrate and quickly polymerized with UV radiation. The inorganic metal oxide layer, mostly aluminum oxide, is deposited via a reactive sputtering process. Typically the organic layers vary between 0.25 and 4 micron in thickness and the metal oxide layers between 30 to 100 nm. What is really unique about this process is that the organic phase is deposited as a liquid: the film is very smooth locally (< 2 Angstrom variation) and also has extremely good planarizing properties over particles and high topographical structures like “cathode separators”, “ink jet wells”, and Active Matrix pixel structures. So while the local flatness creates an ideal surface for growing an almost defect free inorganic layer, the liquid takes care of covering topography. It should also be mentioned that while even non-conformal methods to deposit oxides, like CVD, have difficulty covering cathode separators without creating voids, they also struggle to coat often more then 4 micron high structures in an acceptable process time.
The multilayer provides redundancy and since the remaining defects in the inorganic layers are few and far in between and not connected, a very long diffusion path to the substrate results as well. The organic layers also provide a function of stress release layer in thermal shock testing. An extensive model for the diffusion through this type of barriers has been developed by G Graff et al. The main findings of this study are that
i) High quality inorganic films coupled with a multilayer architecture are necessary to achieve OLED barrier requirements (large spacing between defects).
ii) Lag time (transient diffusion), not steady state flux, dominates gas permeation in these multilayer thin films systems.
iii) Consideration of steady state, alone, is not sufficient to describe and predict the performance of multilayer barrier films one must consider the transient regime.
The Vitex Barix(TM) process has been shown to meet telecommunication application specifications for a wide variety of OLED displays: passive and active matrix displays, bottom, top and transparent displays, and it works equally well for small molecule, polymer and phosphorescent OLEDs.
Figure 2 - Schematic presentation of the process steps of the Barix encapsulation
Born in Amsterdam, The Netherlands. Bachelors and PhD in Theoretical and Physical Chemistry at University of Leiden , The Netherlands. Joined the Philips Research Laboratories in 1984, worked on high resolution photolithography and plasma etching. In 1990 became Department head of the group "Polymers and Organic Chemistry" and headed Polymer LED activities since 1991. The success of this work lead in 1998 to the creation of a new business with a manufacturing line: Philips Components, PolyLED. After a half a year as general manager, continued his work as innovation manager and CTO for Emerging Displays in this new business, situated in Heerlen, The Netherlands. Joined Vitex in San Jose as of March, 2002 as Chief Technology Officer.
Tuesday, February 12
Michael's Restaurant at Shoreline Park
2960 N Shoreline Blvd
Mountain View, CA 94043
6 PM social hour
7 PM dinner
8 PM lecture
$30 with advance registration
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broiled salmon with a lemon buerre blanc
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