The most frequently asked question that I get is, “Why is my barrier coating hard to improve?” Like the coatings, the answer can vary from simple for very basic barrier coatings to much more complex for higher-barrier performance coatings.
It is worth noting just how thin the vacuum-deposited coating usually is that we expect to be a perfect barrier. Typically, the metal is on the order of 20 nm – 40-nm thickness, which is less than 1/1000th the thickness of a human hair and less than 1/100th the thickness of aluminum foil. At this thickness the metal coating is too thin to be self-supporting, and it relies on the substrate to support the coating. The barrier performance achieved by the metal, or transparent oxide, coating is dependent on the perfection of this very thin coating. Any break in the coating or hole through the coating will reduce the barrier performance, and the bigger the size and greater the number of these defects, the worse the barrier performance will be.
Contributors to barrier defects
We can separate out some different contributors to what can cause these defects. Among them are the incoming materials; the vacuum-deposition system; and, in particular, the cleanliness and cleaning protocol for the system; the deposition process; and finally film handling throughout the film and coated film’s lifetime.
The incoming materials are the polymer film and the aluminum wire. The polymer web will have surface contamination where any particles can potentially cause pinholes in the deposited coating, and low molecular weight organic contamination, such as oligomers, can cause reduced adhesion and pick-off pinholes. The aluminum wire will have an oxide layer on the surface and, if the wire is old, this can be thicker than for fresh wire, thus the increased oxide proportion can lead to an increase in spitting during the thermal-evaporation process resulting in pinholes in the coating.
Coater operational causes
The way in which the vacuum system is operated can affect the quality and level of defects in the coating. Cleaning the system is essential to minimize the vacuum pressure during deposition. Stray coating onto shields will build up and increase the surface area considerably, and this coating can absorb water vapor, which will extend the pump-down time and limit the base pressure. Cleaning the shields will generate vast quantities of particles of which the larger particles will fall out of the atmosphere quickly whereas the small particles will remain in the atmosphere for minutes to hours. Cleaning shields in-situ will result in many more particles remaining inside the vacuum system, which have the potential to increase film contamination, that in turn can increase the number of coating defects.