Specific Process Knowledge/Characterization/Stress measurement/Stress origins

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This page is written by Rebecca Ettlinger, a member of the DTU Nanolab staff

Stress dependence on film and growth characteristics

In the page overview page on stress measurement you can find links to studies made at DTU Nanolab about specific deposition results with our own sputter and e-beam evaporation systems. This page aims to give more general information about the origin of stress in thin films. It appears that the following is a good rule of thumb, though it does not completely match the concrete observations for the sputtered films referred to above:

Tensile stress tends to increase when there is low atomic mobility and high deposition rate Compressive stress tends to increase when there is high atomic mobility and low deposition rate

This means the stress depends both on inherent material properties and on deposition conditions.

Examples of materials with comparatively high atomic mobility include Au, Cu, and Ag. They will tend to exhibit more compressive stress.

Examples of materials with comparatively low atomic mobility include Cr, Ni - and Pt??. They will tend to exhibit more tensile stress.

First-hand evidence of the differences in stress in these materials is that Cr and Ni layers in combination with other metal layers in the PVD equipment at Nanolab cause a lot of flaking.

You can increase the atomic mobility (and reduce the tensile stress) by increasing the temperature. In sputtering you can decrease the pressure and add a bias to accelerate the sputtered atoms towards the growing film for increased mobility. In evaporation you could add mobility to the growing film by using an increased substrate temperature or by ion bombardment during the deposition.

In addition, all other things being equal the tensile stress decreases/compressive stress increases with smaller grain size. The grain size tends to increase for thicker layers, meaning that the tensile stress will tend to increase when a thicker layer is grown. This is apparently especially true for the many metals with low mobility that tend to form columnar grains during thin film growth.

Models of thin film growth and stress

The text above is based on a model of thin film stress developed by E. Chason and collaborators at Brown university. You can read about it in their many publications including this tutorial.

The main explanation for the origin of tensile stress in thin films is that these films start growing as islands rather than as a continuous atomic layer. The surface energy of these islands is reduced when islands grow together, which makes up for the energy needed to exhibit tensile stress.

The origin of the compressive stress is controversial but might have to do with grain growth and densification of the film, especially in columnar growth of materials that are capped by an oxide layer.

An interesting observation also seen here at Nanolab is that stress in thin films can change significantly over time after the end of the deposition. In some materials this occurs over the course of days not just minutes - apparently this can, e.g., take place in nickel and in gold, see, e.g., articles about TiPtAu and TiPt contacts on III-V materials.

To learn more generally about stress in thin films, we recommend the classical article by John A. Thornton and D. W. Hoffman from 1979, Stress-related effects in thin films, and other articles by the same author. The tutorial by E. Chason refers back to the model of thin film growth developed in that paper.

'Note this page is in progress - comments are very welcome.'