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MIT Electrical Engineering and Computer Science
EECS Event |
Tuesday, February 27, 2001
2:00 PM
Grier Room, Room 34-401A
Abstract
Electroless deposited cobalt based thin films, Co0.9W0.02P0.08, Co0.9P0.1 and Co0.96W0.04 , were investigated as diffusion barriers for copper metallization in advanced ULSI devices. Electrochemical deposition can be dominated by reaction kinetics resulting in a nanocrystalline or amorphous metallic layer. This microstructure is suitable for application as a diffusion barrier as it eliminates the direct, fast diffusion paths via the grain boundaries. In electroless deposition of the cobalt based alloys, nanocrystalline h.c.p cobalt grains were obtained. The average grain size is related to the amount of phosphorus and tungsten incorporated in the film.
When this non-equilibrium structure was subjected to thermal treatments, TEM and XRD studies revealed phenomena of microstructural evolution such as nucleation, crystallization, grain growth along a preferred (0002) orientation, and phase transformation.
The diffusion barrier quality was characterized by capacitance versus voltage and capacitance versus time measurements of MOS structures. These evaluations indicate that 30 nm thick films can function as effective barriers against copper diffusion up to 450*C. The failure mechanism of the barriers at this temperature is attributed to grain boundaries diffusion of copper. The relative quality as diffusion barriers of the electroless cobalt based films is explained by the average grain size compared to the barrier thickness and by grain boundaries stuffing.