Q: Why is my vacuum-deposited barrier coating hard to improve?
By Dr. Charles A. Bishop
A: 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 thickness is on the order of 20 nm to 40 nm, which is less than 1/1,000th 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 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 potentially can 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
How the vacuum system is operated can affect the quality and level of coating defects. Cleaning the system is essential to minimize the vacuum pressure during deposition. Stray coating onto shields will build up and increase the surface area, and this coating can absorb water vapor, which will extend the pump-down time and limit base pressure. Cleaning the shields will generate vast quantities of particles; the larger ones will fall out of the atmosphere quickly, whereas 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 can increase film contamination. That, in turn, can increase the number of coating defects.
The deposition process can, in theory, produce a wide variety of variations in the coating, but in reality, there are few parameters that can be varied. Most systems are run as fast as possible, and so the deposition rate, process pressure and substrate temperature have very little latitude for variation. The problems that can occur with the deposition process relate to spitting from the molten pool of metal in the resistance-heated boat. The minority elements in the aluminum wire and the surface oxide all contribute to the slag or “crud” that forms on the surface of the molten pool. This crud tends to migrate to the edges of the molten pool, and it is this that results in spitting. If the wire feed varies or the power to the resistance-heated boat varies, the molten pool will expand or contract, and as it moves over the boat surface, the crud at the edges can be ejected as spits. The more constant the power and wire feed, the fewer spits that will occur.
Film handling before the deposition process can generate an electrostatic charge on the surface and can attract atmospheric particles to the surface. Following the deposition process, the front-surface contact may move particles on the surface, which can leave behind pinholes in the coating. Some vacuum systems have been built without any front-surface contact rolls, but this only hides the problem because the pinholes may be generated on the next system on which the film is wound.
Cleanliness is next to product improvement
If we look at each of these sources of coating defects, we can see that there is only a limited opportunity to change the number of defects produced by the deposition process. The places where improvements can be made are in the cleanliness and cleaning of the vacuum system. Using exchange shields and cleaning them in a remote location will help reduce the particle count inside the vacuum system. The other route to improving the barrier performance is to improve the quality of the incoming materials – in particular, the quality of the surface of the incoming polymer film.
Neither of these options for improvement is particularly easy to implement. Both require the surface of the film and coated film to be measured so that any changes can be assessed for effectiveness. Unfortunately, this means that there is no quick fix to barrier improvement.