CSEM Scientific and Technical Report 2008

Batch Fabrication of Ultrathin Nanoporous Membranes
A.-M. Popa, P. Niedermann, R. Pugin
Block-copolymer assisted nanostructuring has been successfully employed for the definition of densely packed nanopores with a mean diameter of
80 nm in silicon nitride thin films with a thickness of 100 nm. The release of these films by standard microfabrication techniques leads to suspended
nanoporous membranes with areas ranging from 100 to 16000 μm 2 .
Nanoporous membranes are essential components in
ultrafiltration, sensing and “smart” cell-culture devices. Most
available systems, however, display poor chemical and
mechanical stability, broad pore size distribution, and a high
aspect ratio of the nanopores (1:1000). These properties
result in fast membrane degradation, poor molecular cut-off,
important material losses, and long response times. In order
to overcome these drawbacks CSEM has recently developed
processes for the fabrication of mechanically stable ultrathin
silicon nitride membranes with extremely narrow pore size
distribution and nanopore aspect ratio close to 1:1 (Figure 1).
Figure 1: Ultrathin nanoporous silicon nitride membrane for size
exclusion based separation of nanoscale molecules and particles
Block-copolymer (BC) assisted nanostructuring is a powerful
technique which allows the definition of dense, hexagonally
packed arrays of nanopores and nanopillars in a wide range of
materials, such as semiconductors, dielectrics and metal
oxides. The approach involves the use of a self-assembled
block-copolymer film as a resist in a dry reactive ion etching
process, thus transferring the nanofeatures of the polymeric
film into the underlying substrate. CSEM has already
demonstrated a variation of the nanopore diameter between
40 and 80 nm as well as a surface coverage between 12%
and 25%, which can be tuned independently [1] .
This clean room compatible method has been up-scaled for
the structuring of 4 inch wafers. Silicon nitride (SiN) is the
material of choice for the fabrication of the ultrathin
nanoporous membranes due to its outstanding mechanical
and chemical stability. A 100 nm thick SiN layer was
deposited by LPCVD on silicon wafers which were previously
coated with thermally-grown silicon oxide. Using BC assisted
lithography, an array of nanopores with diameters of
80 ± 15 nm could then be fabricated into the SiN thin film.
Standard microfabrication techniques were employed for
releasing the nanoporous SiN films. Openings for membrane
areas ranging from 1000 to 16000 μm 2 were defined by
photolithography. Dry reactive ion etching and wet-etching
were successively used for the back etch of the silicon
substrate and the silicon oxide etch-stop layer.
500 μm
Figure 2: 4’’ wafer carrying 100 nm thick Si3N4 nanoporous
membranes; SEM image showing four pyramidal KOH openings
defining the membrane areas.
The insert in Figure 2 shows an SEM image acquired on the
back side of the processed wafer. The typical pyramidal shape
of the openings yielded from the anisotropic KOH etch can be
seen. In the high magnification SEM image shown in Figure 3
a high density of hexagonally packed pores with
homogeneous sizes can be observed. The mean coverage of
membrane surface by pore openings is ~15% of the total area.
Large area membranes (lateral aspect ratio thickness : width
higher than 1:100) could thus be produced without any
microscopic fractures or cracks.
Figure 3: High magnification SEM image of the membrane back-side;
the narrow pore diameter distribution is also visible.
It was checked by FIB cross sections through the released
membrane that the nanopores extend through the entire
thickness of the silicon nitride film with very limited under-cut.
These components are promising for applications in
ultrafiltration and biosensing. First tests as well as their
functionalization with stimuli responsive macromolecules for
the design of smart nanovalves are currently under way. The
financial support of OFES and the European Community via
the European Research Training Network project
BIOPOLYSURF is gratefully acknowledged.
[1] A.-M. Popa, et al., CSEM Scientific Report 2007, page 53
1 μm
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