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with a 2000 nm pitch length and a 1500 nm pit diameter.
An improved version with a 1250 nm pitch and a 1000 nm pit
diameter shows an 800% average sensitivity improvement;
this is the version that has been successfully transferred to a
plastic base.
Two ways to fabricate
In the sheet-level version of the new process, a “positive”
master template is fabricated from silicon by electron-beam patterning, etching, and coating with an anti-stick film (see figure).
The master template is used to form interim “negative” templates from a soft PMMA polymer, which are then UV-cured.
These interim templates are then used in a similar replication
process to form the final “positive” textured surfaces.
In the R2R version of the process, a nickel shim is electroplated on the silicon master, detached, and welded to an
embossing reel. A PMMA carrier web coated with a UV-curable
lacquer is embossed by the reel and simultaneously UV-cured
through the transparent film, then quickly post-cured and laminated with a protective foil. In both cases, the nanostructure is
coated with a gold film.
The two methods show slight differences in the grain structure of the gold layers; the researchers expect that the result will
be slightly different levels of Raman amplification—results that
will be discussed in a future paper. Reproducibility of Raman
measurements for the R2R and sheet-level fabrication processes
are 5% and 9.41%, respectively.
“This development (transfer of the substrate from silicon to
plastic) takes a big step forward in bringing the cost of man-
ufacture down,” says Charlton. “Another advantage is that
we can increase the active area size considerably with little
impact on production cost, which makes them more flexible for
customer applications.”
Charlton notes that the development of the disposable SERS
substrate was undertaken via the FP7 PHOTOSENS project,
which involves a number of other European Union partners.
“We have drawn on the expertise of these partners to get
this to work,” he says. “In particular, Nanocomp (Lehmo,
Finland), 3D AG (Baar, Switzerland), and VTT (Espoo, Finland)
have solved considerable technical issues to make the pattern
transfer to plastic work. We hope to commercialize the new
plastic version of the sensors though our Renishaw and other
project partners (Philips).”
Upcoming research
The researchers plan to work on the specialized surface chemistry
that enables the SERS sensors to be made specific to a particular
molecule. “The longer-term plan is to integrate further functionality into the SERS sensor by incorporating underlying waveguide
structures in the plastic to better couple the incoming pump light,
and to provide dual sensing methodologies,” says Charlton. “For
example, we could combine SERS with photonic-crystal sensors
to provide better quantification of signal data (SERS is notoriously
