Size-independent stresses in Al thin films thermally strained down to −100 °C

Size and temperature dependencies of thermal strains of {1 1 1} textured Al thin films were determined by in situ X-ray diffraction (XRD) in the temperature range of −100 to 350 °C. The experiments were performed on 50–2000 nm thick Al films sputter-deposited on oxidized silicon (1 0 0) substrates. The in-plane stresses were assessed by measuring the {3 3 1} lattice plane spacing at each temperature in steps of 25 °C during thermal cycling. At high temperatures, the films could only sustain small compressive stresses. The obtained stress–temperature evolutions show the well-known increase of flow stresses with decreasing film thickness for films thicker than 400 nm. However, for thinner films, the measured stress on cooling is independent of the film thickness. This lack of size effect is caused by the flow stresses in the thinnest films exceeding the maximum stress that can be applied to these samples using thermomechanical loading down to −100 °C. Thus, the measured stresses of ∼770 MPa in the thinnest film represent a lower limit for the actual flow stresses. The observed stresses are also discussed taking microstructural information and possible constraints on dislocation processes into account.