Stress-induced voiding and stress relaxation in passivated aluminum line metallizations

Abstract

The yield strength of metallic thin films bonded to hard substrates, particularly with an additional passivation, can be significantly higher than is customary for bulk samples of the same metal. This is related to the constrained nature of deformation. The constraints place restrictive conditions on the mechanisms of deformation that produce stress relaxation. Because of this, the stresses are generally high and triaxial, which favors void nucleation. In narrow aluminum based metallizations bonded to ceramic substrates of lower thermal expansion coefficients, constraint effects give rise to exceptionally high tensile stresses subsequent to excursions to elevated temperatures. These stresses relax with time during cooldown, and also thereafter, even at room temperature. The degree of constraint, and thus the ensuing stresses at room temperature, is significantly larger for passivated line metallizations than for unpassivated continuous films or lines. Void growth arises naturally as a stress relaxation mechanism alternative to plastic flow. It is shown that in some hours after cooldown, stress relaxation in pure aluminum lines becomes limited by void growth. In the long run, after about a month, stress evolution and void growth are controlled by the relaxation of grain boundary back stresses by dislocation creep in the bulk.