Effect of the Tantalum Barrier Layer on the Electromigration and Stress Migration Resistance of Physical-Vapor-Deposited Copper Interconnect

Abstract

The effects of a Ta barrier layer on electromigration and stress migration of Cu films are investigated. The Ta barrier layer enhances both the (111) texture and the median grain size of the annealed Cu films. At 225°C, the electromigration mean time to failure (225°C-MTF) of Ta/Cu/Ta multilayer interconnects is about two times longer than that of Cu monolayer interconnects. After 500 thermal cycles, the 225°C-MTF of Ta/Cu/Ta multilayer interconnects does not change and is about three times longer than that of Cu monolayer interconnects. The activation energy Ea of electromigration of Ta/Cu/Ta multilayer interconnects is 0.77 eV, which is higher than that of Cu monolayer interconnects (0.65 eV). Since the Ta/Cu/Ta specimen has enhanced crystallographic texture and larger grain size, it has both higher electromigration endurance and thermal stress resistance than Cu. However, the measured MTF is shorter than that predicted by an equation proposed by Vaidya and Sinha [Thin Solid Films 75 (1981) 253]. The shorter MTF and smaller Ea values for the Ta/Cu/Ta multilayer compared to the predicted ones are due to the weak Ta/Cu interface. A lifetime of 100 years is predicted for Ta/Cu/Ta multilayer interconnects with and without 500 thermal cycle stress.