Interfaces and stresses in thin films
Original Entry by Holly McIlwee AP225 Fall 2009
F. Spaepen, Interfaces and stresses in thin films, Acta mater., 31 – 42, 2000.
Physical vapor deposition, Thin films, Interfaces, Grain boundaries, Stress
In this review Spaepen details the current progress made in studying the effects of interfacial stresses in thin films, specifically, those which are not a result of an applied load or thermal effects. Previous work in this area has focused on single crystal thin films. There is a less complete understanding of these interactions in polycrystalline films. So far grain boundaries have been studied and in this review the direct mechanical action of surfaces and interfaces monitored during vapor deposition is the focus.
The total energy associated with a surface of a thin film under constant strain is given by:
Solving for surface stresses gives:
An interface stress can be characterized in the same way if the strain on either side of the interface is the same. It should be noted that the origins of these stresses are interatomic interactions as well as film structure and dislocations.
Measurement of the films was done under conditions, reducing, as much as possible, additional stresses such as those brought on by mutual solubility of multiple layers. Measurements were taken by three different methods. Regardless of the test method or sample preparation, the results were consistent. Although the fact that the compressive stress that was observed is in contrast to the tensile stress that is calculated for. This discrepancy is still unresolved.
Abermann et al. formulated some of the first theories about the stress in thin films based on material mobility. It was found that there are three stress stages: initial compressive, tensile, and finally a constant incremental compressive stress. 1-4
Thus far this work has focused on semiconductor films and more recently metallic polycrystalline films. It is believe that work so far can be used as a platform for determining stresses in organic thin films, with the assumption that there will be characteristic differences which present themselves.
One example mentioned herein of initial work in studying surface and interface stresses in soft matter thin films is work by Berger et al. The effect of alkyl chain length is studied in relationship to stress in alkanethiol self-assembled monolayers.
In their work Berger et al. have realized that the stress inside a self- assembled monolayer of alkanethiols is under compressive stress and that the stress increases linearly with alkyl chain length.
1. Abermann, R, Kramer, R, and Maser, J, Thin Solid Films, 1978, 52, 215.
2. Abermann, R, and Koch, R, Thin Solid Films, 1986, 142, 65.
3. Abermann, R, Vacuum. 1990, 41: 1279.
4. Thurner, G, and Abermann, R, Thin Solid Films, 1990, 192, 277.
5. Berger, R, Delamarche, E, Lang, H P, Gerber, C, Gimzewski J K, Meyer, E and Giintherodt. H-J Science 1997, 276, 2021.
6. F. Spaepen, Interfaces and stresses in thin films, Acta mater., 31 – 42, 2000.