Lateral grain size enlargement and surface flattening of Cu thin films by using adequate underlayers and by thermal annealing in ultra high vacuum

The effects of body centered cubic (BCC) solid solution underlayers on the metallurgical size and flatness of Cu thin films, fabricated on them, were investigated. As the underlayer materials, Cr₈₀Al₂₀, Cr₉₀Ni₁₀, Cr₈₀Fe₂₀, Fe ₇₅Cr₂₅ and Fe were examined. Lateral grain size of the Cu films fabricated on the Cr-Al, Cr-Ni, Cr-Fe, Fe-Cr under layer was enlarged, comparing to that on the Fe underlayer. The difference between the surface energy (γ_S^X) and the interfacial energy (γ_SL^X-Cu) of these underlayer is close to the surface energy of Cu at the melting point (γ_S ^Cu(T_m)). However, the surface roughness of Cu films simultaneously increased with increasing the lateral grain size. In order to suppress the surface roughening of Cu films, post-thermal annealing procedure was applied under the ultra-high vacuum for the films fabricated on the Cr-Ni and Cr-Ni-Fe underlayers. We finally succeeded to enlarge the lateral grain diameter more than 100 nm for 50-nm-thick Cu films, maintaining a fiat surface (R_a = 0.32 nm).