Void formation in gold films on yttrium-doped zirconia in the initial stage of de-wetting

Thin gold films on single-crystal Y2O3-doped ZrO2 (YSZ) de-wet from the substrate during thermal annealing. Formation of voids in the gold film at the Au/YSZ interface is found to be the key mechanism of de-wetting. This is in contrast to the mechanism usually accepted for de-wetting of metallic films on oxides, which commences at the free surface. This paper concentrates on the initial stage of the void formation. High-resolution transmission electron spectroscopy (HRTEM) measurements provide details of the atomic layers structure. They reveal that the voids appear in association with grain boundaries and that the gold atomic layer at the Au/YSZ interface is not of a low index. An angle of 5° exists between the low index (1 1 1) plains of gold and the surface (0 0 1) plain of the YSZ substrate. The HRTEM measurements also reveal the ordering of the atomic layers at the void boundaries. It is concluded that the high energy of the gold interface layer in combination with grain boundary elimination drives the formation of the voids and that the high energy of the gold interface layer is the reason why de-wetting originates from voids at the Au/YSZ interface rather than from grooving at the gold-free surface. Voids and hillocks formation are shown to be two independent phenomena.