MS-3-O-1891 Solid-state Dewetting of Thin Films Studied by in situ Transmission Electron Microscopy

A major issue of thin films is their instability against dewetting at elevated temperatures resulting from the energetically unfavorable configuration in comparison to a set of droplets or particles. This phenomenon can occur at temperatures well below the melting temperature of the bulk material and is denominated as solid-state dewetting [1].
Two different views on solid-state dewetting have developed from applications: Dewetting of mainly metallic or semiconducting thin films poses a degradation mechanism on today's electronics, magnetics and optics applications. It can occur at elevated temperatures in application or even during fabrication causing critical failure. On the contrary controlled dewetting has lately been employed to fabricate ordered arrays of nanoparticles. Different approaches were developed to influence order, shape and spacing, e.g. by the use of structured substrates.
In both cases a thorough understanding of the underlying mechanism of solid-state dewetting is necessary. Surface self-diffusion has generally been accepted to be the main transport mechanism [1], but a recent work showed the importance of grain boundary diffusion and arose doubt whether this generalization can be made [2]. A common way to hinder solid-state dewetting is the use of alloyed thin films. It is however poorly understood, how this influences the mechanism giving rise to the higher stability [3].
In this work we apply advanced in situ transmission electron microscopy (TEM) techniques to study the phenomenon of solid-state dewetting. Au thin films on silicon nitride substrates have been chosen as model system. A DENSsolutions sample heating system is used for in situ heating experiments capable of heat treatments of up to 800 °C at very low drift rates.
Fig. 1 exemplarily shows ADF-STEM images of such an experiment. On the left the as deposited Au thin film is shown, whereas the image on the right shows the thin film after 50 min at 300 °C. The phenomenon of solid-state dewetting is clearly visible. It has to be mentioned, that the as deposited Au thin film was not continuous but already featured voids. This has been exploited to study directly the process of void growth separated from the process of void nucleation.
Fig. 2 underlines the solid-state character of the observed process. It shows subsequent HRTEM images of a bridge between two Au islands at 300 °C at an advanced stage of the dewetting process. The retraction of a surface step can be observed, while the permanent observation of lattice fringes shows the solid-state character of the material.

1. Thompson, Annu. Rev. Mater. Res. 42 (2012), pp. 399-434
2. Kovalenko et al., Acta Mater. 61 (2013), pp. 3148-3156
3. Müller et al., J. Appl. Phys. 113 (2013), 094301