Hillock formation of Pt thin films on Yttria stabilized Zirconia single crystals

TL;DR

This study investigates the formation of hillocks in 50 nm Pt thin films on yttria stabilized zirconia, revealing that deposition method influences morphology, stress relaxation causes hillock formation, and anisotropic energies are characterized.

Contribution

It demonstrates how deposition method affects hillock formation, links hillock morphology to stress relaxation, and quantifies anisotropic energies in Pt thin films on zirconia.

Findings

Anisotropic pyramidal hillocks form instead of holes during annealing.

Hillock formation is driven by stress relaxation within the film.

Anisotropic step free energy and kink energy are quantified.

Abstract

The stability of a metal thin films on a dielectric substrate is conditioned by the magnitude of the interactive forces at the interface. In the case of a non-reactive interface and weak adhesion, the minimization of free surface energy gives rise to an instability of the thin film. In order to study these effects, Pt thin films with a thickness of 50 nm were deposited via ion-beam sputtering on yttria stabilized zirconia single crystals. All Pt films were subjected to heat treatments up to 973 K for 2 h. The morphological evolution of Pt thin films has been investigated by means of scanning electron microscopy (SEM), atomic force microscopy (AFM) and standard image analysis techniques. Three main observations have been made: i) the deposition method has a direct impact on the morphological evolution of the film during annealing. Instead of hole formation, that is typically observed as response to a thermal treatment, anisotropic pyramidal shaped hillocks are formed on top of the film. ii) It is shown by comparing the hillocks' aspect ratio with finite element method (FEM) simulations that the hillock formation can be assigned to a stress relaxation process inside the thin film. iii) By measuring the equilibrium shapes and the shape fluctuations of the formed Pt hillocks the anisotropy of the step free energy and its stiffness have been derived in addition to the anisotropic kink energy of the hillock's edges.