Mechanical characteristics of aluminum thin films on silicon and gallium arsenide

In situ stress measurements during thermal cycling from room temperature to 400 °C have been performed for thermally evaporated aluminum films with various thicknesses on silicon and gallium arsenide substrates. The as-deposited Al films on both substrates exhibit a tensile stress which significantly relaxes with time at room temperature. They exhibit a compressive stress relaxation behavior upon heating, followed by a tensile stress yielding phenomenon at the elevated temperature and a tensile stress relaxation at the ambient temperature during cooling. The compressive stress relaxation owing to the formation of hillocks is triggered when the stress reaches a certain level. The required threshold stress can be significantly lower if the relaxation occurs at a higher temperature. The films deposited on GaAs show a higher crystallinity than those on Si. This explains why they exhibit a higher yield stress, a higher critical stress, and a steeper stress-temperature curve during cooling than their counterparts deposited on Si. These mechanical characteristics are also dependent on the film thickness, i.e. they are stronger if the films are thinner. They apparently become stronger in the second cycle, plausibly resulting from the increment of dislocation density in the first cycle.