Thermal Cycling and Surface Reconstruction in Aluminum Thin Films

Abstract

The appearance of "annealing hillocks" on heat treated aluminum films has been previously reported. Thin films of Al, deposited on passivated wafers, were annealed for various lengths of time in the temperature range 200°–500°C. The effects of grain size, alloying and surface passivation on hillock formation, were studied. It is the purpose of this report to summarize the effect of thermal cycling on similar films and to show in more detail the size and appearance of the hillocks. Aluminum was deposited on oxidized silicon substrates in a vacuum of about 10−8 Torr. Grain sizes at the deposition temperature were on the order of <1μ, 1μ, and 5μ. Manual cycling was carried out between a hot stage in N2 and a room temperature heat sink. Control samples were annealed (uncycled) for equivalent times‐at‐temperature. Photomicrographs of the surface at its various stages of reconstruction were taken and scanning electron micrographs provided a more detailed view of individual hillocks and their surroundings. At the higher temperatures (425°C) thermal cycling increases the number of hillocks by factors of 2 or 3, with the typical hillock density estimated to be approximately 106/cm2. Cycling also increases the size of the protrusions, and hillocks as high as 3.5μ have been observed growing out of a 0.7μ film. Experiments employing glass passivation confirm earlier results that surface reconstruction can be eliminated.