Copper Interconnects: Surface State Engineering to Facilitate Specular Electron Scattering

In situ electron transport measurements on epitaxial 10-nm-thick Cu(001) with Al and AlOx cap layers indicate that the electron surface scattering specularity increases when the density of electronic surface states decreases. The Cu layers were sputter deposited on MgO(001) substrates, and their resistance is measured in situ as a function of the Al cap thickness d
_Al
= 0-1.4 nm as well as oxygen exposure from 10
^-1
to 10
⁵
Pa·s. The resistance increases with increasing d
_Al
, indicating a decrease in the surface scattering specularity that is well described by an exponential decay, attributed to the linearly increasing number of localized surface states available for electron scattering. In contrast, exposure to 0.04-50 Pa O
₂
results in a decrease in the measured resistance, as the Al cap is oxidized to form AlO
_x
, leading to a decreasing surface density of states and increasing scattering specularity with increasing x. The conductance enhancement is most pronounced for d
_Al
= 0.4 nm since a larger d
_Al
leads to incomplete Al oxidation, while a smaller d
_Al
insufficiently suppresses copper surface oxidation, causing a transition to diffuse surface scattering. The overall results indicate that insulating liner layers provide a conductance advantage for narrow interconnect lines.