Low Energy Ion Assisted Vapor Deposition

The performance of multilayered thin film materials often depends sensitively upon the (physical) roughness and degree of (chemical) mixing at interfaces. Irradiation of a growth surface with an assisting ion beam is often used to modify surface roughness. Molecular dynamics has been used to explore the use of low energy (less than 20 eV) Xe+ and Ar+ assisted deposition of model Ni/Cu/Ni multilayers to control both physical roughness and chemical mixing. The study indicated that under normal ion incidence condition, ion energies as low as 3 eV could effectively flatten the relatively weakly bonded copper surface (and therefore the nickel on copper interface). Higher ion energies (at least 10 eV) were required to flatten the more strongly bonded nickel surface. Chemical intermixing by an exchange mechanism between a surface atom and an underlying atom in an already deposited layer depended upon the binding energy of the already deposited layer. As a result, significant chemical mixing occurred as 9 eV (and above) ions impacted with nickel atoms on an already deposited copper surface. At a given ion incident energy, (the heavier) Xe÷ ions resulted in less roughness but more mixing. A modulated ion assistance strategy in which no assisting ion beam was used while depositing the first few monolayers of each new metal layer was found to successfully reduce both interfacial roughness and interlayer mixing.