Less is more: improved thermal stability and plasmonic response in Au films via the use of sub-nanometer Ti adhesion layers.

The use of a metallic adhesion layer is known to increase the thermo-mechanical stability of Au thin films against solid-state dewetting, but in turn results in damping of the plasmonic response, reducing their utility in applications such as heat-assisted magnetic recording (HAMR). In this work, 50 nm Au films with Ti adhesion layers ranging in thickness from 0.1 - 5 nm were fabricated, and their thermal stability, electrical resistivity, and plasmonic response were measured. Sub-nanometer adhesion layers are demonstrated to significantly increase the stability of the thin films against dewetting at elevated temperatures (>200oC), compared to standard adhesion layer thicknesses that are in the range of 2 - 5 nm. For adhesion layers thicker than 1 nm, diffusion of excess Ti through Au grain boundaries and subsequent oxidation was determined to result in degradation of the film. This mechanism was confirmed using transmission electron microscopy and X-ray photoelectron spectroscopy on annealed 0.5 nm and 5 nm adhesion layer samples. The superiority of sub-nanometer adhesion layers was further demonstrated through measurements of the surface-plasmon polariton resonance; those with thinner adhesion layers possessed both a stronger, spectrally sharper resonance. These results have relevance beyond HAMR to all Ti/Au systems operating at elevated temperatures.