Electromigration of Cu/low dielectric constant interconnects

Electromigration in damascene Cu/low dielectric constant interconnects with overlayers of CoWP, Ta/TaN, SiN_x or SiC_xN_yH_z and Cu(Ti) interconnects capped with SiN_x 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 SiN_x or SiC_xN_yH_z. 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 SiN_x or SiC_xN_yH_z, and Cu(Ti) bamboo lines capped with SiN_x, 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.