research activities in 2007 - CSEM

High Aspect Ratio Nanopores in MEMS Compatible Substrates
A.-M. Popa, R. Pugin, M. Liley
One of the objectives of the European project Biopolysurf is the design and fabrication of smart nanovalves for biology-related applications. The
work presented below concerns the production of nanoporous thin films that will be subsequently functionalized with temperature responsive
macromolecules for the fabrication of smart freestanding nanoporous membranes.
The fabrication of nanoporous membranes is of considerable
interest for many applications including ultrafiltration, osmosis,
prevention of particle emission, high throughput DNA
sequencing and biosensing.
The fabrication of 2D nanopore arrays in ultrathin
(freestanding) membranes can be accomplished by standard
top-down fabrication techniques including Nano Imprint
Lithography, Electron Beam and Focus Ion Beam; however,
these techniques are currently still limited to structuring very
small areas, and therefore, to prototyping. An alternative
approach is the well-known bottom-up approach, where to
scale up the structures one takes advantage of the tendency
of molecular building blocks to self-organize. The bottom-up
methods used for the fabrication of ultrathin films generally
include Langmuir-Blodgett deposition, layer-by-layer
assembly, sol-gel or various biomimetic techniques; however,
only few approaches such as colloidal and block-copolymer
lithography have allowed the fabrication of 2D nanopore
arrays in ultrathin membranes for industrial applications.
Recently, CSEM has developed a silicon based freestanding
nanoporous membrane in which the pore diameter can be
tuned between 10 and 20 nm. The membrane is 60 nm thick
and has been fabricated by combining standard
microfabrication processes and block-copolymer lithography
(a nanostructured block-copolymer thin film is used as etch
mask for the transfer of the self-assembled structure into the
underlying material by deep reactive ion etching). With this
technology, the formation of pores with aspect ratios up to 1:5
could be demonstrated [1] . The nanoporous membranes are
fabricated with a support structure which makes their
manipulation and integration in macroscopic devices
straightforward.
Based on these first results and with the main objective to
improve pore size distribution and the mechanical robustness
of the membrane, a new clean room compatible and wafer
scale applicable etching process has been developed for
thicker silicon based nanomembranes. Once spin-coated, the
block-copolymer thin film is directly used to pattern an
intermediate metal hard mask. This additional step allows to
be reached a much higher selectivity of the etching process
(slower degradation of the mask) resulting in the formation of
pores with aspect ratios higher than 1:10 into a silicon
nanomembrane. The nanoporous thin membranes fabricated
in this way are shown in Figure 1.
According to Scanning Electron Microscopy (SEM) analysis,
the pore channels are parallel throughout the membrane
(lateral undercutting has been avoided using highly
anisotropic DRIE process) and are densely packed in a
hexagonal configuration respecting the initial micelles selfassembled
structures. The pore size distribution is very
narrow and the mean coverage of membrane surface by pore
openings is ~20% of the total area.
Figure 1: Scanning Electron Microscopy (SEM) images (top views
and transverse sections) of silicon nanoporous substrates with 80
and 40 nm diameter pores.
Another big advantage of this approach is the tunability of the
pore size and filling factor (ratio of pores to solid area), which
are both directly linked to the polymeric micellar mask in terms
of diameter and inter-particle distance, respectively [2] . The
next step will be the release of the nanoporous membranes
using standard microfabrication techniques. Subsequently, the
suspended membranes will be modified with temperature
responsive polymers, in order to achieve the foreseen
reversible valve function within the pores. Applications such
as ultrafiltration or biosensing are currently under
investigation.
This work was partly funded by the OFES and the European
Community via the European Research Training Network
project BIOPOLYSURF. CSEM thanks them for their support.
[1] A. Hoogerwerf, et al., “Reinforced Nanoporous Membranes”,
CSEM Scientific and Technical Report 2006, page 41
[2] S. Krishnamoorthy, R. Pugin, H. Heinzelman, J. Brugger,
C. Hinderling, Adv.Funct.Mat., 16, (11), (2006), 1469-1475
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