Solid films are usually metastable or unstable in the as-deposited state, and they will dewet or agglomerate to form islands when heated to sufficiently high temperatures. This process is driven by surface energy minimization and can occur via surface diffusion well below a film's melting temperature, especially when the film is very thin. Dewetting during processing of films for use in micro- and nanosystems is often undesirable, and means of avoiding dewetting are important in this context. However, dewetting can also be useful in making arrays of nanoscale particles for electronic and photonic devices and for catalyzing growth of nanotubes and nanowires. Templating of dewetting using patterned surface topography or prepatterning of films can be used to create ordered arrays of particles and complex patterns of partially dewetted structures. Studies of dewetting can also provide fundamental new insight into the effects of surface energy anisotropy and facets on shape evolution.

Solid-State Dewetting of Thin Films

Dense polycrystalline thin films of ZrO2 (3 and 8 mol % Y2O3) were produced by the pyrolysis of zirconium acetate precursor films, which were deposited on single crystal Al2O3 substrates by spin-coating aqueous solutions of zirconium acetate and yttrium nitrate. Dense films were heat treated to encourage grain growth. With grain growth, these films broke into islands of ZrO2 grains. Identical areas were examined after each heat treatment to determine the mechanism that causes the polycrystalline film to uncover the substrate. Two mechanisms were detailed: (a) for a composition which inhibited grain growth and produced a polycrystalline film with very small grains, the smallest grains would disappear to uncover the substrate, and (b) for a composition which did not inhibit grain boundary motion, larger grains grew by enveloping a smaller grain and then developed more spherical surface morphologies, uncovering the substrate at three grain junctions. In both cases, the breakup phenomenon occurred when the average grain size was larger than the film thickness. Thermodynamic calculations show that this breakup lowers the free energy of the system when the grain-size-to-film-thickness ratio exceeds a critical value. These calculations also predict the conditions needed for polycrystalline thin film stability.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0884291400008529

The instability of polycrystalline thin films: Experiment and theory

This study investigates the dependence of Ag resistivity on film thickness during temperature ramping as a means to access thermal stability. In situ van der Pauw four-point probe analysis is used to determine the onset temperature; the temperature when the electrical resistivity deviates from linearity during the temperature ramp. At that point, the silver thin films become unstable due to void formation and growth during thermal annealing. The thermal stability of Ag thin films on SiO2 in a vacuum is greatest when thicknesses are greater than 85 nm. Using an Arrhenius relation in terms of onset temperature and film thickness, an activation energy (0.32±0.02 eV) for the onset of agglomeration in Ag thin films on SiO2 ramped at a rate of 0.1 °C/s is determined. This value is consistent with the activation energy for surface diffusion of silver in a vacuum, which is believed to be the dominant mechanism for agglomeration of silver thin film.

Thickness dependence on the thermal stability of silver thin films

Hillock formation, hole growth and agglomeration in thin silver films

A model for hillock growth in thin films such as those of lead and tin is developed. It is proposed that hillock growth is a form of stress relaxation in which atomic species diffuse along the film‐substrate interface to the base of the hillock. The latter extrudes out along the grain boundaries connecting it to the rest of the film. The stability and kinetics of this process are considered. Taking into account Nabarro‐Herring diffusion creep as a parallel stress relaxation mechanism observations such as an incubation period for hillock growth and the effects of film thickness on hillock density are accounted for. In addition, observations on lateral growth of hillocks and unusually tall hillocks (whiskers) are interpreted within the framework of the present model.

D.L. Trimm

Papers

Hillock formation by surface diffusion on thin silver films

Kinetics of hillock and island formation during annealing of thin silver films

The role of microstructure and surface energy in hole growth and island formation in thin silver films