A sandblaster at the atomic level

Date:
September 20, 2021
Source:
Vienna University of Technology
Summary:
Modifying surfaces by shooting particles at them - this technique,
called 'sputtering', is indispensable in surface science. However,
if the surface is not perfectly smooth and regular, it is hard
to predict the result of the sputtering process. Scientists have
now managed to explain the effect of particles on rough surfaces
during sputtering - with implications for fusion research and
even astrophysics.



FULL STORY ==========================================================================
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 much smaller, on an atomic scale. The surface is irradiated with ions, i.e. charged atoms, allowing microscopic impurities
to be removed, for example.


==========================================================================
If you are dealing with perfect surfaces where all the surface atoms are arranged exactly in a smooth plane, established theoretical models can
predict the effects of ion bombardment quite easily. But in practice,
this is very rarely the case. For complicated, rough surfaces, it is
difficult to say how much material will be removed during sputtering. A computational model developed by researchers from TU Wien now makes it
possible to characterize the surface roughness in a simple way and thus correctly describe the sputtering process even for more complicated
samples.

Removing or depositing thin layers "Sputtering of surfaces by
ion bombardment is a very popular and versatile technique," says
Prof. 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, one must understand the sputtering process in great detail.

The same applies to nuclear fusion research: In the search for extremely resistant materials for the inner wall of a future fusion reactor, one
must be able to calculate how much material is removed from the reactor
chamber by the constant bombardment with high-energy ions. This also
provided the original motivation for this study, which was funded by the European fusion research program EUROfusion and also involved colleagues
from Uppsala University, the Helmholtz Center in Dresden and the Max
Planck Institute for Plasma Physics in Greifswald. The investigated
effects are also important in Astrophysics, where rock surfaces, for
example on the moon or on the planet Mercury, are bombarded by the
charged particles of the solar wind and thus eroded and changed by
sputtering processes.

It's the impact angle that matters "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, first author of the current study. "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:
Some areas of the surface are not 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 the sputtering.

Very differently rough surface samples were examined in Vienna. Using
modern high-resolution microscopy methods, the roughness of the
samples was first analyzed, then they were bombarded with ions and the experimental results were compared 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." How high the individual
elevations are on the 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 Christian 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.

========================================================================== Story Source: Materials provided by Vienna_University_of_Technology. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. C. Cupak, P.S. Szabo, H. Biber, R. Stadlmayr, C. Grave,
M. Fellinger, J.

Bro"tzner, R.A. Wilhelm, W. Mo"ller, A. Mutzke, M.V. Moro,
F. Aumayr.

Sputter yields of rough surfaces: Importance of the mean surface
inclination angle from nano- to microscopic rough regimes. Applied
Surface Science, 2021; 570: 151204 DOI: 10.1016/j.apsusc.2021.151204 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/09/210920121744.htm

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