|Ph.D Student||Shaffir Eran|
|Subject||Process of De-Wetting and Beading of Gold Films on Single|
Crystals Yttria Stabilized Zirconia (YSZ)
|Department||Department of Physics||Supervisor||Professor Emeritus Ilan Riess|
|Full Thesis text|
Many thin metal films on single ceramic crystals de-wet from the substrate during thermal annealing in the solid state and form small crystalline particles. This issue is critical for the durability of thin film devices. In this work the de-wetting and agglomeration of 200nm and 20nm thick Au films deposited on single crystals of Y2O3-doped ZrO2 (YSZ) were investigated.
The main accepted theory to date explains solid state de-wetting as a result of thermal grooving through the vertical grain boundaries of the film, starting at the free surface. We find faceted voids that nucleate at the metal-ceramic interface, and then grow to form faceted pinholes in the film. Thus void nucleation is identified as the main mechanism for solid state de-wetting in gold on YSZ, rather than grain boundary grooving at the free surface of the film.
In parallel to void nucleation, bubbles form in the film driven by pressure exerted by gases chemisorbed on the substrate. These bubbles explode and collapse yielding additional holes in the film.
The holes expand via diffusion driven by minimization of the surface and interface energies. In a later stage the holes coalescence to form elongated and winding holes. Finally isolated islands are formed that eventually yield small single crystals.
Void nucleation and expansion is theoretically explained by the tendency of the system to minimize the Gibbs-free energy. This minimization occurs by the elimination of grain boundaries and stress. A small value of work of adhesion between the substrate and the film is an essential condition for void nucleation.
Grain growth is detected at all the de-wetting and agglomeration stages. In the case of the 20 nm film a secondary grain growth is observed. In the case of the 200nm films, the grains grow in a way which is not typical of either normal or secondary grain growth.
Hillocks formation is also detected at an early stage of the film de-wetting. Although part of the material from the voids is probably accumulated in the hillocks, it seems that hillock and void growth are independent processes. Hillock formation is explained by non- uniform stress relief that occurs due to the higher expansion coefficient of the Au film in comparison to the YSZ substrate.