CSEM Scientific and Technical Report 2008

Silicon Nanostructuring for Controlling Surface Wettability
N. Blondiaux, E. Scolan, A.-M. Popa, J. Gavillet • , R. Pugin
CSEM reports on the fabrication of sub-micrometer silicon pillars with controlled aspect ratios by combining thin polymer film structuring and dry
etching. The structures were used to create superhydrophobic surfaces. Depending on the aspect ratio of the pillars, different superhydrophobic
wetting states were observed. Above a certain aspect ratio, the resulting surfaces have particularly good self-cleaning properties.
The effect of surface roughness on wettability has received
increasing research interest during the last decade. An
accurate control of surface topography can indeed
dramatically enhance the wetting properties [1] . Depending on
their surface chemistry, rough surfaces will become either
“superhydrophilic” or “superhydrophobic”. From a
technological point of view, many potential applications of
these effects have been demonstrated. Superhydrophilic
surfaces are for instance being used for their anti-fogging
properties while super-hydrophobic surfaces are interesting
for their outstanding self-cleaning properties. The most wellknown
example to illustrate the latter effect is found in nature
with lotus leaves which present very intricate surface micro
and nanostructures.
The objective of this work is to create superhydrophobic
silicon surfaces by both surface structuring and surface
functionalization. The dimensions of the structures have been
systematically varied in order to investigate the effect of
topography on wettability.
Figure 1: SEM image of high aspect ratio silicon pillars
Initially, silicon nanostructures were fabricated by combining
self-assembly with standard microfabrication processes. A
self-assembled structured polymer film was prepared by
polymer demixing and then used as an etch-mask for the
pattern transfer into the underlying silicon substrate by
reactive ion etching. The lateral size of the structures was
controlled via the control of the polymer self-assembly process
while dry etching duration provided an excellent control over
the height of the structures. A wide library of structures was
thus achieved thanks to the tunability of both structuring
processes; an example of a high aspect-ratio structure is
presented in Figure 1. The surface was then made
superhydrophobic by post depositing a low surface-energy
material on the silicon pillars using either PVD process or by
means of wet silanisation.
For all structures, water did not spread but beaded up over the
surface as can be seen on the photograph presented in
Figure 2.
54
Figure 2: Photograph of water droplet deposited on superhydrophobic
surfaces
Depending on the aspect-ratio of the structures, different
wetting behaviors have been observed. The differences were
analysed by measuring the angle at which water droplets
would start rolling off the surface. Figure 3 shows the rolling
angle as a function of droplet volume [2] . For small aspect-ratio
structures, water droplets were sticking on the surface as
emphasized by the large rolling angles measured. Above 1:1
aspect-ratio, the surfaces became self-cleaning and very low
rolling angles were observed for all droplet volumes.
Figure 3: Effect of the volume of water droplets on their rolling angles
for surfaces with different aspect ratio pillars
These superhydrophobic surfaces may find applications in the
fabrication of hydrophobic and self-cleaning MEMS
components, and for surface-engineering for biotechnologies.
CSEM thanks the EC (www.napolyde.org) for their financial
support.
•
LITEN, Commissariat à l'Energie Atomique (CEA), Grenoble,
France
[1] Li, et al., Chem. Soc. Rev., 36 (2007), 1350–1368
[2] Rolling angle is the tilting angle of the substrate when a drop
deposited upon it starts to roll downward