I have been looking back through my archive of books, journals and conference proceedings, and the thought struck me that, in the area of roll-to-roll (R2R) vacuum deposition and, in particular, aluminum vacuum deposition, it has been some time since I have seen anything new reported. I have seen reports of technology that was tried but did not expand into commonplace use some 20-30 years ago suddenly appear again. Over the last few years and possibly much longer, there have been many incremental changes reported but nothing that I would recall as being a step change to allow for a significant increase in system performance.
Numerous process improvements
Comparing R2R vacuum metallizers from 30-40 years ago to the present day, it is clear that the basic process is the same, but it is also evident that there have been improvements to almost every aspect of the process. The web handling is much more precise, and the winding quality is better. The web cooling has improved because of using the gas wedge/injection between the web and drum to increase the heat transfer coefficient. Boat quality is better, and boat-life has increased. Monitoring of the quality, uniformity and thickness of the coating, including defects has improved, and the pumping efficiency has been enhanced as well. Other improvements used only for niche-market applications such as the quality and resolution of pattern metallizing also has occurred.
This list is quite short considering it spans ~40 years and, if I am honest, the rate of improvement does appear slow and modest. I understand that the system manufacturers sell small numbers of systems each year, and the systems have to be competitive on cost, and so there is a limited budget to spend on development. This, in turn, leads to a delay in the manufacturers’ improvements becoming public knowledge as they tend not to talk about any advantageous development in the hope that they can maintain this competitive advantage. When the improvement becomes known, this is followed by their competitors developing an equivalent improvement, which they are prepared to publicize.
This means that changes across the industry take many years before they become the expected baseline for any new system. Probably the last of this type of development was the deposition of transparent aluminum oxide (AlOx) using a process developed using a standard aluminum metallizer. This was first done more than 20 years ago since which time each of the system suppliers have developed their own version of the process.
This does not mean that the aluminum-deposition process is without limitations. The resistance-heated evaporation boats still have a limited lifetime and as so many are used, particularly as the web widths have gradually increased, there is always a risk of one failing during a deposition run, giving rise to loss of product. The use of multiple open-evaporation sources across the web width also leads to limitations of material efficiency and deposition uniformity. Alternative deposition sources have been produced, such using magnetic levitation of the aluminum and so not requiring any crucible or enclosed=evaporation sources that enable improved uniformity and very high material efficiency, but these alternatives have had other process problems that have deemed them an unattractive replacement.