Heavy-ion induced adhesion of thin gold films to oxidized substrates of tantalum and silicon

Abstract

Energetic heavy ion beams are capable of enhancing the adhesion of metallic films to a variety of substrates. Gold films (200–600 Å) evaporated onto substrates of tantalum and silicon (with native oxides) were bombarded with ions of 12C, 16O, 28Si, 35Cl and 58Ni at 2.85 MeV/nucleon. The threshold dose required to produce a peel strength greater than the Scotch-tape peel strength for a gold surface was measured as a function of ion species and angle of incidence. We observed the threshold dose to vary as the cosine of the angle with respect to normal incidence. The dependence on particle type for the AuTa system (with approximately 40 Å native oxide) was found tobe D th(cm −2) = 10 17( dE dx ) Au −3.0±0.2 for all particle beams where ( dE dx ) Au is the (MeV cm 2/mg) of the ion in gold. A substrate of 6000 Å of tantalum oxide gave identical results. The value of the exponent, −3.0±0.2, differs significantly from the value −1.6±0.2 reported earlier by Tombrello et al. for AuTa. The AuSi system is described by D th = 6 × 10 18( dE dx ) Au −4.1±0.3 . Auger analysis of the AuTa interface suggests a migration of the nativ result of ion bombardment. For the AuTa system, some irradiated regions that passed the Scotch-tape test just after bombardment were observed to fail when the test was repeated a few days later. Thicker gold films and higher doses resulted in longer adhesion times. These and other observations suggest that the post-irradiation decline of adhesion for AuTa is caused by diffusion of ambient atoms through the gold film rather than by an intrinsic time dependence of the adhesive forces.