Interconnect metals beyond copper: reliability challenges and opportunities

Reliability challenges of candidate metal systems to replace traditional Cu wiring in future interconnects are discussed. From a reliability perspective, a key opportunity is electromigration improvement: due to their high melting point and slower self-diffusion kinetics, higher current carrying capabilities are possible. Also, the higher cohesive energy and better resistance to oxidation of some metals potentially allows for barrierless integration, although adhesion properties must be carefully optimized. Besides avoiding small grain pinning and enabling high aspect ratio trench fill, the main processing challenges are identified to be a) avoiding seam voids, b) adhesion, c) CMP and d) disruptive metal etch. Main reliability challenges are related to higher mechanical stresses and higher joule heating which could lead to delamination during further processing and packaging and to enhanced electromigration in nearby metal lines.