Electromigration of Cu/low dielectric constant interconnects

Electromigration in damascene Cu/low dielectric constant interconnects with overlayers of CoWP, Ta/TaN, SiNx or SiCxNyHz and Cu(Ti) interconnects capped with SiNx was studied. The results showed that the migration fast path in the bamboo-like lines primarily occurred at the interface. Cu lines fabricated with various forms of TaN/Ta liner including PVD TaN, ALD TaN, and PVD body centered cubic α- or tetragonal β-Ta liners were also investigated. Both thin surface layers of CoWP or Ta/TaN and the addition of Ti in the Cu lines significantly reduced the Cu/cap interface diffusivity and remarkably improved the electromigration lifetime when compared with Cu lines capped with SiNx or SiCxNyHz. Activation energies for electromigration were found to be 1.9–2.4 eV, 1.4 eV, 0.85–1.1 eV, and 1.3 eV for the bamboo-like Cu lines capped with CoWP, Ta/TaN, and SiNx or SiCxNyHz, and Cu(Ti) bamboo lines capped with SiNx, respectively. The structural phase of the Ta was found to have an insignificant effect on the Cu mass flow rate. A large via size, thicker liner and/or stable connected exposed liner can provide a longer lifetime and tighter lifetime distribution, at the expense of chip density or effective Cu line conductivity.