22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Surface modification of metals induced by high fluxes of low energy helium ions
I. Tanyeli1, L. Marot2, D. Mathys3, M.C.M. van de Sanden1 and G. De Temmerman4
1
Dutch Institute for Fundamental Energy Research (DIFFER), De Zaale 20, 5612 AJ Eindhoven, the Netherlands
2
Department of Physics, University of Basel, Klingelbergstrasse 82, 4056 Basel, Switzerland
3
Centre of Microscopy, University of Basel, Klingelbergstrasse 50/70, 4056 Basel, Switzerland
4
ITER Organization, Route de Vinon sur Verdon, CS 90 046, 13067 St. Paul lez Durance Cedex, France
Abstract: Several metal surfaces, such as iron, titanium, aluminum and copper, were
exposed to high fluxes (in the range of 1023 m-2s-1) of low energy (<100 eV) Helium (He)
ions. Different surface modifications, such as fiber like structures, voids and nano pillars,
are observed on these metals. The differences and similarities in the development of
surface morphologies are discussed in terms of the material properties and compared with
the results of similar experimental studies.
Keywords: surface modification, He ions
1. Introduction
Surface structuring by energetic ion bombardment has
been widely studied and considered as an efficient surface
processing technique.
Different materials can be
processed by this technique, so-called ion beam
sputtering. As a particular case, the interaction of helium
ions with metal surfaces, especially with tungsten, has
long been studied extensively because of helium
production in fusion reactors [1, 2]. Recently, significant
surface modifications on tungsten under low energy He
ion irradiation has been reported [3]. These studies
revealed the formation of fine nanostructures exhibiting a
high porosity of up to 90% [4]. In this study, we explore
the effect of low-energy helium ion exposure of several
metal surfaces, such as iron (Fe), titanium (Ti), aluminum
(Al) and copper (Cu).
2. Experimental
Samples were exposed to pure helium plasma in PilotPSI, a high-flux linear plasma generator. The plasma is
generated by a thermal plasma (cascaded arc) source and
confined by an axial magnetic field. The plasma density
profile has a Gaussian shape and the maximum ion flux
was in the range of 2-7×1023m-2s-1 during these
experiments. The samples are negatively biased to
control the ion energy.
Polycrystalline samples, which are 20 mm in diameter
and 1 mm in thickness, were mechanically polished with
SiC grinding papers and followed by 3 and 1 µm diamond
and 0.05 µm alumina suspensions. During plasma
exposure, the peak temperature was measured by a
multiwavelength pyrometer (FMPI SpectroPyrometer,
FAR Associates), which measures in the wavelength
range of 900-1600 nm. In addition, an infrared camera
(FLIR A645 SC) was used to measure the 2D surface
temperature profile.
The surfaces were analyzed by high resolution scanning
electron microscopy (SEM, Hitachi S-4800 field emission
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at 5 kV). For cross-sectional imaging, the samples were
prepared by focused ion beam (Dual Beam FIB/SEM)
milling method.
3. Results and Discussion
Fig. 1 shows the surface modifications on iron samples
after high flux (3-4×1023 m-2s-1) of low energy (~25 eV)
He ion irradiation. Fiber like iron nanostructures are
formed at surface temperatures of 450-700 °C [5]. A
network like structure is observed on the surface at
around 450 °C (see first column of Fig. 1). With the
increase in the surface temperature, nanosized structures
start to appear on the surface. The characteristic size of
these structures increases with temperature. The results
indicate that surface processing by high flux of low
energy He ion bombardment provides a size controlled
nanostructuring on iron surfaces, which is consistent with
the results from tungsten and molybdenum surfaces [6].
This could indicate that similar nanostructure growth
mechanism is valid for tungsten, molybdenum and iron.
However, physical sputtering has to be taken into account
during the discussion on nanostructuring of iron, since
mass loss was measured after the plasma exposures. The
effect of sputtering is neglected due to high sputtering
threshold energy for W and Mo by He ions.
Titanium exposures do not show any nanostructure
growth on the surface.
No significant surface
modification was observed until 400 °C. Beyond 600 °C,
voids are detected in the cross-section images of titanium
(see Fig. 2). Low sputtering yield, which was calculated
relying on the mass loss measurements, and highly
packed crystal structure of titanium might be the reasons
that one could not observe nanostructure growth.
Pillar like structures were formed on aluminum and
copper surfaces after He plasma exposures as seen in
Figs. 3 and 4. These structures show similarity with the
self-organized nanopatterns and nanodots obtained by ion
beam sputtering. The surface modifications that we
1
Fig. 1. Evolution of He induced nanostructure formation
on iron surface for three different exposure times with ion
flux of 3.5-6.5×1023 m-2s-1 [5].
Fig. 3. (a) SEM image (52° tilt) of Al surface which was
irradiated by He ions with ion energy of 35 eV with
surface temperature of 250 ° C and (b) a cross sectional
image which was taken from the region seen in (a) (white
layer seen on top is Pt which was coated during FIB
milling).
Fig. 2. Cross-section images of Ti sample (exposed for
10 minutes with surface temperature of 750 ° C) taken
under low (a) and high magnification (b).
observe on Al and Cu cannot be only attributed to
sputtering caused by ions. The effect of He clustering and
consequently void formation on surface modification is
clearly seen on Al and Cu surfaces (see Figs. 3 and 4).
Both He ion irradiation and physical sputtering would be
considered as effective factors in the morphology changes
of Cu and Al.
4. Conclusion
The effect of helium plasma treatment of several metal
surfaces has been studied. A controlled nanostructure
formation on iron has been achieved. Different surface
modifications are experienced for different metals, such
as titanium, aluminum and copper. The morphological
changes has been discussed by considering the effects of
2
Fig. 4. Cross-section images of samples, which are
exposed to helium plasma with surface temperatures of
(a) 500 ° C and (b) 650 ° C.
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void formation underneath the surface and physical
sputtering.
5. Acknowledgment
This work is part of the research programme of the
Stichting voor Fundamenteel Onderzoek der Materie
(FOM), which is financially supported by the Nederlandse
Organisatie voor Wetenschappelijk Onderzoek (NWO). It
is supported by the European Communities under the
contract of Association between EURATOM and FOM
and carried out within the framework of the European
Fusion Programme.
6. References
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N. Yoshida. J. Nucl. Mater., 283-287, 1134 (2000)
[2] H. Ulmaier. Nucl. Fusion, 24, 1039 (1984)
[3] K. Tokunaga, et al. J. Nucl. Mater., 337-339, 887
(2005)
[4] S. Kajita, T. Saeki, N. Yoshida, N. Ohno and
A. Iwamae. Appl. Phys. Express, 3, 085204 (2010)
[5] I. Tanyeli, L. Marot, M.C.M. van de Sanden and
G. De Temmerman. ACS Appl. Mat. Interfaces, 6,
3462 (2014)
[6] G. De Temmerman, et al. J. Vac. Sci. Technol. A,
30, 041306-6 (2012)
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