Fabrication of a porous polyimide membrane using a silicon nanowire array
as a template
Woong Kim
⁎
, Myung-Ki Lee
Department of Materials Science and Engineering, Korea University, Seoul 136-713, South Korea
abstractarticle info
Article history:
Received 10 October 2008
Accepted 17 January 2009
Available online 26 January 2009
Keywords:
Nanomaterials
Polymers
Porosity
Membranes
Nanowires
Polyimide
We demonstrate that a porous polyimide membrane can be fabricated by curing liquid polyimide on a
vertically oriented silicon nanowire array and selectively etching away the nanowire-array-template using
xenon difluoride (XeF
2
). Pore size and density using the described technique are controllable. The former is
dependent on nanowire diameter and the duration of etching, whereas pore density is determined by silicon
nanowire density. We believe that the described porous membrane fabrication method can be applied to
various polymer and nanowire systems.
© 2009 Elsevier B.V. All rights reserved.
1. Introduction
Porous polymer membranes have many applications in biotech-
nology and electronics for the separation and/or filtration of gases,
biomolecules, and environmentally hazardous materials, and as in-

embedded nanowires. The membra ne was then slightly etched with O
2
plasma (20 min at 1 00 W in a 50 sccm O
2
stream) to expose the silicon
nanowire tips. A u nanoparticles at the tips of the nanowires were then
etched using potassium iodide and iodine (KI/I
2
) solution, and the
substrate was thoroughly rinsed with deionized (DI) water. To selectively
etch the silicon nanowires and lea ve the polyimide membr ane intact,
XeF
2
etching was c arried out over 5 0–200 cy cles of exposure t o 4 Torr of
XeF
2
and 2 Torr of N
2
for 60 s . Polyimide membr anes were e ither
detached from silicon substr at es aft er this stage o r were detached by
dipping substr at es in buffered hydrofluoric acid (BHF) so lution. F or
scanning ele ctron microscopy (S EM) characterizations, a bout 5 nm of Au
was sputter ed onto the poly imide membranes produced.
3. Results and discussion
The procedure used to fabricate the porous polyimide membranes
is summarized in Fig. 1.Asafirst step, silicon nanowires were
epitaxially grown on Si (111) substrates by CVD. Since nanowires grow
preferentially in theb111Ndirection under the conditions used, they
were oriented vertically to the substrate. An SEM image of an angled
view of the vertically aligned silicon nanowires is shown in Fig. 1a.

The film becomes extremely thin at the nanowire tips as shown in
Fig. 2b.
To expose nanowires, the polyimide membrane was etched with
O
2
plasma. Etching conditions, e.g., power, duration, and O
2
flow rate,
were optimized to expose only the upper portions of nanowires
without appreciably damaging the supporting polyimide. An SEM
image of the exposed portion is shown in Fig. 1c. Having exposed the
nanowires, the Au nanoparticles at the nanowire tips were removed
using gold etchant (KI/I
2
) solution [10].
Finally, the silicon component of nanowires was selectively
removed using XeF
2
as etchant. Etch rate of polyimide is negligible
Fig. 3. Silicon nanowires with various diameters; (a) ~50, (b) 90, (c) 150, and (d) 220 nm. Nanowires were synthesized from gold nanoparticles with diameter of ~ 30, 50, 100, and
150 nm, respectively.
Fig. 4. SEM images of porous membranes with various pore sizes; (a) ~800 nm, (b) 1 μm, and (c) 1.5 μm. Pore sizes are dependent nanowire diameter, which in the present study were
90, 150, and 400 nm, respectively. Pore size is dependent on etching time, e.g., (a) ~800 nm at 200 cycles vs. (d) ~170 nm at 50 cycles.
935W. Kim, M K. Lee / Materials Letters 63 (2009) 933–936
compared to that of silicon [10]. Fig. 4b shows the top surface of a pore-
containing polyimide membrane after this selective etching process.
As mentioned earlier, membrane pore density is wholly dependent on
nanowire density, which is in turn determined by gold nanoparticle
density . Moreover , there is usually a near one to one r elation between
nanowire and gold nanoparticle numbers [11]. Fig. 2 shows two

inc rease the ranges of the pore diameters and densitie s formed.
4. Conclusions
Our studies indicate that silicon nanowires can be used as
sacrificial templates for the fabrication of porous polyimide mem-
branes. Moreover, since the densities and diameters of silicon nano-
wires can be easily adjusted, membranes can be fabricated with pre-
determined pore densities and sizes. In the present study, membranes
were successfully fabricated with pore diameters ranging from
170 nm to 1.5 μm and densities ranging from 0.1 to 1 pore/10 μm
2
.
Furthermore, the novel fabrication technique described can be applied
to the fabrication of porous membranes from different polymers and
template nanowires comprising different materials.
Acknowledgement
This work was supported in part by the Korea Science and
Engineering Foundation through the Pioneer Converging Technology
Program (No. M10711160001-08M1116-00110).
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