Sputtering of surfaces by ion bombardment. Image: TU Wien.
Sputtering of surfaces by ion bombardment. Image: TU Wien.

If you want to remove a layer of paint from a metal surface, you can use a sandblaster – countless grains of sand are blasted onto the surface, and what emerges is clean metal. 'Sputtering' can be imagined in a very similar way – only at much smaller, atomic scales. Rather than grains of sand, the surface is irradiated with ions, i.e. charged atoms, allowing microscopic impurities to be removed, for example.

When dealing with perfect surfaces where all the surface atoms are arranged exactly in a smooth plane, established theoretical models can quite easily predict the effects of ion bombardment. In practice, however, this is very rarely the case. For complicated, rough surfaces, it is difficult to determine how much material will be removed during sputtering. Now, researchers at the Vienna University of Technology (TU Wien) in Austria have developed a computational model that can characterize surface roughness in a simple way and thus correctly describe the sputtering process even for complicated samples.

"Sputtering of surfaces by ion bombardment is a very popular and versatile technique," says Friedrich Aumayr from the Institute of Applied Physics at TU Wien. "On the one hand, it can be used to remove material very precisely, for example in semiconductor technology, to create perfectly clean surfaces. On the other hand, however, it can also be used to selectively evaporate any material, which is then deposited on another surface; for example, to produce super-reflective eyeglass lenses or hard material coatings on special tools." To use the right amount of material in this process, the sputtering process needs to be understood in great detail.

The same applies for nuclear fusion research. In the search for extremely resistant materials for the inner wall of a future fusion reactor, researchers need to calculate how much material will be removed from the reactor chamber by constant bombardment with high-energy ions. This actually provided the original motivation for this study, which was funded by the European fusion research program and involved colleagues from Uppsala University in Sweden, and the Helmholtz Center in Dresden and the Max Planck Institute for Plasma Physics in Greifswald, both in Germany.

Similar understanding is also important in astrophysics. This is because rock surfaces on planetary bodies such as the Moon and Mercury are bombarded by the charged particles of the solar wind. The surfaces are thus eroded and changed by this sputtering processes.

"The amount of material knocked out of the sample surface by ion bombardment depends on two main things besides the projectile energy: the angle at which the ions hit the surface and the roughness of the surface," says Christian Cupak from TU Wien, who is first author of a paper on this work in Applied Surface Science. "We were looking for a way to characterize the roughness of the surface in such a way that you can infer exactly how much material is removed during sputtering."

Surface roughness changes the local impact angle of the particles and there are also shadowing effects, resulting in some areas of the surface not being hit by ions at all. In addition, the removed material may be re-deposited in certain places, much like debris in mountainous terrain. This further reduces the effectiveness of sputtering.

The researchers examined a variety of rough surfaces. They used modern high-resolution microscopy methods to analyze the roughness of these surfaces and then bombarded them with ions, comparing the experimental results with the model calculations. "In the end, we succeeded in determining a single parameter that describes the sputtering process very reliably," says Christian Cupak. "It is a measure of the average surface inclination."

They found that the height of individual elevations on a rough surface does not play a significant role. A roughness on the nanometer scale has quite similar effects to a roughness on the order of millimeters, as long as the angular distribution of the individual surface pieces is the same in both cases.

"The question is not how high the average mountain is on the surface, but merely how steep it is," explains Cupak. "We were able to show that our parameter describes the final outcome of the sputtering process much better than other roughness parameters that have been used so far."

The research team at TU Wien will now use the new surface characterization method in both fusion research and astrophysical studies. In industrial applications, the new modeling method could provide greater reliability and precision.

This story is adapted from material from TU Wien, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.