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 characteristics

In the page about stress dependence on sputter parameters for Si, Ni, and Cu you can read some recommendations about which sputter parameters led to which stress characteristics in specific materials.

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

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 and Ni - and Pt and Ru??. They will tend to exhibit more tensile stress.

The differences in stress in these materials is definitely something we can recognize from first-hand experience at Nanolab, as Cr and Ni layers in combination with other metal layers in the PVD equipment cause a lot of flaking.

You can increase the atomic mobility (and reduce the tensile stress) by increasing the temperature, and in sputtering you can decrease the pressure and add a bias to accelerate the sputtered atoms towards the growing film. In evaporation you could potentially add energy to the growing film by using ion bombardment apart from increased deposition temperature.

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

These observations are 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 associated with tensile stress appearing.

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

All this also points to an interesting observation: Stress in thin films can change significantly over time after the end of the deposition. In some (high mobility) materials this might be over the course of days not just minutes - apparently this can e.g. take place 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 also 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.'