Stability of nano-scaled Ta/Ti multilayers upon argon ion irradiation

The effects of argon ion irradiation on structural changes in Ta/Ti multilayers deposited on Si wafers were investigated. The starting structures consisted of sputter deposited 10 alternate Ta (∼23 nm) and Ti (∼17 nm) layers of a total thickness ∼200 nm. They were irradiated at room temperature with 200 keV Ar⁺, to the fluences from 5 × 10¹⁵ to 2 × 10¹⁶ ions/cm². The projected ion range was around mid-depth of the multilayered structure, and maximum displacements per atom ∼130. It was found that, despite of the relatively heavy ion irradiation, individual nanocrystalline Ta and Ti layers remain unmixed, keeping the same level of interface planarity. The changes observed in the mostly affected region are increase in lateral dimensions of crystal grains in individual layers, and incorporation of bubbles and defects that cause some stretching of the crystal lattice. Absence of interlayer mixing is assigned to Ta-Ti immiscibility (reaction enthalpy ΔH_f = +2 kJ/mol). It is estimated that up to ∼5 at.% interface mixing induced directly by collision cascades could be compensated by dynamic demixing due to chemical driving forces in the temperature relaxation regime. The results can be interesting towards developing radiation tolerant materials based on multilayered structures.