Electromagnetic levitation: A new technology for high rate physical vapour deposition of coatings onto metallic strip

For about three decennia, there has been a growing interest from the strip coating business in fully continuous or semi-continuous PVD based coating processes. For the deposition of thin coatings (∼ < 100 nm) on polymer webs and thin metal foils, magnetron sputtering has become the predominant technology. However, the deposition rate obtained with magnetron sputtering is too low to be economically interesting for the metal sheet and strip businesses. Here, the high rate electron beam (EB) PVD seems to become the state of the art technology, probably together with jet vapour deposition (JVD) for the high vapour pressure or low boiling point metals like zinc and magnesium. This paper presents a novel PVD process developed at Corus, which may alleviate some of the disadvantages and should combine the advantages of both EB PVD and JVD. The process is based on the well-known technology of levitation of conductive materials in high frequency electromagnetic fields. By proper induction coil design high power densities can be achieved and metals with low vapour pressures like aluminium, nickel and copper, as well as alloys can easily be evaporated. The vapour produced can be guided to the substrate by a properly designed vapour distribution system, which achieves excellent coating uniformity and a very high vapour utilisation. The proof that every single step of this novel concept works is presented in this publication. Moreover, extra advantages of this technology i.e. the creation of a plasma during evaporation, are discussed. Further up scaling of this technology, together with Posco, should demonstrate the technical and economical feasibility for application in the metal strip coating business. This technology may also find very valuable applications in other fields of PVD coating e.g. in coating of non-flat surfaces.