Physical, chemical and electrical characterisation of the diffusion of copper in silicon dioxide and prevention via a CuAl alloy barrier layer system

Abstract

Cu and CuAl alloy (90%:10% wt) films deposited on thermally grown SiO2 were studied using thermal stress testing in vacuum, N2 flow and atmosphere ambient all at a temperature of 500°C, in order to rigorously test the effectiveness of the incorporation of Al into a Cu film at preventing degradation of the metal layer and diffusion of Cu into the underlying SiO2 dielectric. Capacitance voltage testing of metal-oxide-semiconductor (MOS) devices, using the Cu and the CuAl alloy as the gate metal show the failure of the Cu reference set via diffusion of Cu at the metal / SiO2 interface in contrast to the stability of the CuAl alloy devices, even following a 500°C anneal in atmosphere ambient. The flatband voltage of the Cu reference MOS structures were altered by the application of an external applied voltage bias, consistent with the diffusion and mobility of Cu+ ions into the underlying SiO2 layer. Optical and scanning electron microscopies of the surface metal layers show the degradation and delamination of the pure Cu films throughout the experimental anneal stages, in contrast to the stability of the CuAl alloy. X-ray photoelectron spectroscopy shows the growth of Al oxide at the surface of the CuAl alloy following thermal anneal which acts to passivate the film, preventing Cu oxide formation which was identified in the pure Cu reference sample set. Transmission electron microscopy analysis shows the inter-diffusion of Cu at the Cu / SiO2 interface following anneal, in contrast to the CuAl alloy which shows the growth of a continuous barrier layer interface. Time-of-flight secondary ion mass spectroscopy (ToF-SIMS) was used to profile the entire metal / SiO2 / Si stack via 3 dimensional reconstructions, showing the inward diffusion of Cu within the Cu control samples and containment of Cu within the metal layer of the CuAl alloy samples.