|Ph.D Thesis||Department of Materials Science and Engineering|
|Supervisors:||Prof. Eizenberg Moshe|
|Prof. Yosi Shacam|
This research aims at understanding how ultra-thin metallic films can retard diffusion of another metal in order to serve as an effective diffusion barrier. Polycrystalline thin metallic films fail due to rapid diffusion through defect paths such as grain boundaries. This research examines how the structure of ultra-thin metallic films can be modified in order to improve barrier integrity.
This aim is technologically relevant to interconnect metallization in ultra-large-scale integrated microelectronic devices. The transition to Cu interconnects requires ultra-thin conductive diffusion barriers deposited by a method characterized by excellent step coverage.
This research focuses on structural modifications of a passive metallic film, namely a metal that is immiscible and does not react with the diffusing metal. A model system was chosen of electroless deposited Co-based films alloyed with P and W as a diffusion barrier against Cu.
Electroless deposition is proposed as a novel method for obtaining diffusion barriers, because it is a highly conformal, selective, low temperature process readily integrated with the industrial Cu electroplating.
Barrier integrity was found to improve because of the structural effect of adding the P and W alloying elements to Co. These elements have very limited solubility both in Co and Cu. However, electroless deposition enables to incorporate them in the Co film at concentrations considerably larger than their equilibrium solubility. Following thermal annealing, the excess P and W in the polycrystalline Co thin film enrich the grain boundaries. This research quantitatively correlates between barrier integrity, structure, and Cu diffusivity through the Co(P,W) films. Thus, it was demonstrated that Co(P -8 at.%, W -2 at.%) and Co(P -10 at.%) films are effective barriers because Cu grain boundary diffusion is hindered due to occupation of boundary sites by the alloying elements.