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

Evaluating the mechanical properties of thin films on thick substrates is tricky. Some useful techniques for mechanical property measurements on macroscopic film specimens exist, e.g., internal stress measurements by x-ray diffraction or by wafer buckling. Many conventional techniques, however, such as indentation for hardness measurement or scratch testing for evaluation of adhesion or wear resistance, yield results that are difficult or impossible to evaluate in terms of more fundamental film properties. In fact, these techniques are more practical engineering tools than tools for scientific study.One solution is to miniaturize the test probe to dimensions approaching the film thickness. The nanoindentation technique for submicrohardness measurement described elsewhere in this issue by Pharr and Oliver is a recent example of such an approach. Still, for very thin films it is difficult to separate the influence of the substrate properties from the film properties, and the indentation process is inherently too complex to be well suited for determining fundamental materials parameters.The rapidly evolving field of micromechanics offers some new possibilities in thin film characterization. Important film properties such as internal stress, elastic moduli, plastic yield limit, and fracture data can be extracted from experiments with micromachined test structures in the 10–1,000 μm size range.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0883769400041646

Mechanical Characterization of Thin Films by Micromechanical Techniques

Effects of grain orientation on hillock formation and grain growth in aluminum films on silicon substrates

Microstructure evolution during dewetting in thin Au films

Platinum and ruthenium oxide (RuO2) deposited by ion beam sputter-deposition are evaluated for use as electrodes for PZT thin film capacitors. The effect of deposition temperature, film thickness, and the presence of oxygen on hillock formation in platinum is discussed. It is shown that the hillock density in Pt/Ti/SiO2/Si films can be significantly reduced by properly controlling the processing conditions and film thickness. Stress measurements correlate with the experimental observation that depositing thinner platinum films (<800 A) is an effective means of reducing hillock formation. The use of an intermediate deposition temperature 200–250°C also helps minimize hillock formation. Diffusion of the Ti adhesion layer into and/or through the platinum was significantly reduced by replacing the Ti with a TiOx adhesion layer. RuO2 electrodes are compared to Pt in terms of resistivity, surface morphology, microstructure and film orientation.

Electrodes for ferroelectric thin films

Hillocks, small outgrowths on a film surface, form when compressional stresses in an aluminum film are relaxed at elevated temperature (≥90 °C), for instance during the phase of rising temperature in an annealing cycle. This paper reports a study of hillock formation in Al films of thicknesses in the interval 0.25–2.2 μm and which have been deposited by electron beam evaporation. Hillock sizes, shapes, number and formation temperatures were determined, the latter on a heating stage in situ in a scanning electron microscope. The internal structure of the hillocks was studied by cross‐sectional transmission electron microscopy technique. These studies provided strong support for the idea that hillocks are formed by migration of material along grain boundaries, presumably at triple junctions, up to the surface where it is deposited in a growing hillock. Initially, the hillocks are separated from the original film surface by a grain boundary‐like interface, but prolonged annealing will cause underlaying grain...

A transmission electron microscopy study of hillocks in thin aluminum films

This article reports a systematic study of local stress relaxationeffects in thin Alfilms,deposited on silicon substrates by sputtering or electron beam evaporation. The stresses were induced thermally during an annealing cycle, as well as thermomechanically by micromechanical technique. The latter experiments were performed i n s i t u in the vacuum chamber of a scanning electron microscope(SEM), which made it possible to monitor the reconstruction of the film surface as it happened, and simultaneously register applied forces, strains, and temperatures. Three fundamental, local stress relaxation phenomena were observed. Compressive film stresses were, at elevated temperature, relaxed by hillock formation. The i n s i t u experiments in the SEM, combined with cross‐sectional transmission electron microscopy of hillock structures, provided important information on the growth mechanism of hillocks. Tensional film stresses at elevated temperature were relaxed by two modes of local stress relief. One mode of tensional stress relief occurred in films of thickness 1 μm or more. Singular Al grains were observed to ‘‘collapse’’ and become wider and thinner than neighboring grains. Such grain collapses are scarcely reported by previous workers, and were in the present investigation seen to occur very suddenly, in a second or less, as the strain and the temperature reached critical levels. A collapse mechanism, based on local plastic yield and interfacial slip, is suggested. Films of submicron thickness relaxed by a second mode: hole formation. Essentially two types of holes were observed: small, circular holes and large branched‐out holes. A hole formation mechanism, based on creep cavitation and interfacial slip, is suggested. The observed holes resemble the ‘‘voids’’ many other workers have reported in narrow, passivated Al lines. The present results show that passivation coatings and narrow geometries are not necessary prerequisites for holes in Alfilms; a high tensional film stress is enough.

Local stress relaxation phenomena in thin aluminum films

Hole formation in thin aluminium films under controlled variation of strain and temperature

A special stress relaxation effect in thin aluminum films evaporated onto oxidized silicon wafers is reported. The effect is observed at elevated temperature and under tensional stress. It appears as if certain Al grains in the film surface suddenly ‘‘collapse’’ compared to surrounding grains. The phenomenon is observed during the cooling phase of an annealing cycle, or when imposing external strain on the film by bending of the substrate at elevated temperature. Such external strain was imposed by micromechanical technique in situ in a scanning electron microscopy (SEM), and the collapse phenomenon was monitored as it happened. The phenomenon occurred instantaneously and only in films of thickness 1 μm or more; thinner films relaxed by hole formation. The grain collapse effect is characterized, and a physical explanation is given, supported by SEM and transmission electron microscopy (TEM) investigations and surface profile measurements.

Nils Kristensen

Papers

A transmission electron microscopy study of hillocks in thin aluminum films

Local stress relaxation phenomena in thin aluminum films

Hole formation in thin aluminium films under controlled variation of strain and temperature

Grain collapses in strained aluminum thin films