The deposition process can, in theory, produce a wide variety of variations in the coating but, in reality, there are few parameters that 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 edge of the molten pool, and it is this than 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 the crud at the edges as it moves over the boat surface 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. By 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 are 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.