Relationship between substrate bias and microstructure in magnetron-sputtered AlCu films

Abstract

The relationship between substrate bias and microstructure in 0.6 microm thick AlCu films deposited from a target containing Al−4wt.%Cu (Al−1.7at.%Cu) by magnetron sputtering has been studied. In-depth copper distributions in films deposited at 0, −100 and −200V r.f. were profiled by Rutherford backscattering spectroscopy (RBS), and corresponding microstructures were characterized by transmission electron microscopy. The copper distribution in the grounded film shows a 1.9at.% plateau region near the film-substrate interface. By contrast, the copper distribution in the biased film shows a peak at the interface, with the copper concentration increasing with bias. In the grounded film, θ-Al 2Cu precipitates approximately 0.2 microm in diameter, identified by energy-dispersive X-ray analysis and selected area diffraction, are located at grain boundaries, whereas smaller θ precipitates are distributed within grains. In biased films, θ precipitates approximately 0.06 microm in diameter are located on the interface. At higher bias level, the number of precipitates increases, whereas the width of columnar grains decreases. The existence of interfacial θ precipitates in biased films provides the microstructural evidence for copper segregation at the interface as determined by RBS, and this may lead to serious problems in reactive ion etching. The formation of interfacial θ precipitates is caused by the effects of ion bombardment and rising temperature induced by substrate bias as sputtering proceeds. All films exhibit a predominant texture with {111} crystallographic planes parallel to the interface.