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Diffusional Hillock Formation in Al Thin Films Controlled by Creep

Published online by Cambridge University Press:  10 February 2011

Deok-kee Kim,
William D. Nix,
Eduard Arzt,
Michael D. Deal and
James D. Plummer
Deok-kee Kim
Affiliation:
Center for Integrated Systems, Stanford University, Stanford, CA
William D. Nix
Affiliation:
Department of Materials Science and Engineering, Stanford University, Stanford, CA
Eduard Arzt
Affiliation:
Max-Planck-Institut für Metallforschung, Stuttgart, Germany
Michael D. Deal
Affiliation:
Center for Integrated Systems, Stanford University, Stanford, CA
James D. Plummer
Affiliation:
Center for Integrated Systems, Stanford University, Stanford, CA
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Abstract

Thermal hillocks in sputter-deposited Al films have been studied as a part of a broad study of stress-induced diffusional processes in Al. Trace amounts of the impurities Ti, W, and O were incorporated into the films during deposition, causing them to be much stronger than most sputter deposited Al films. Stress measurement during thermal cycling, using the wafer curvature method, showed that these Al films are very strong; this finding was corroborated by hardness measurements. Microstructural studies using TEM and FIB showed that the hillocks start to form at the Al/SiO2 interface and grow under the original Al film, with its columnar grain structure. In some cases, the film fails as hillocks grow completely through the original film. The Al film on top of the hillocks appears to inhibit hillock growth by creating a back pressure associated with power law creep of the film. We modeled this form of hillock formation by modifying the boundary conditions in Chaudhari's hillock model [1]. Our model describes hillock formation by diffusion of Al atoms from the surrounding area into isolated hillocks, assuming that the original Al film on top of hillocks deforms following power law creep. Our model can be applied to many different situations by using different creep laws for the top Al film.

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References

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