Page 306 - Optofluidics Fundamentals, Devices, and Applications

280 Cha pte r T w e l v e

12-1-2 Fabrication Methods
The emergence of optofluidic devices is largely enabled by the recent
advances in microfabrication, microfluidics, and polymer processing
technologies [6,7]. The methods of choice are micromachining, soft
lithography, and embossing techniques, which enable the fabrication
of micron-scale fluidic channels in silicon, glass, polymer, and elas-
tomer materials.
Polymers have been accepted as the material of choice for the
integration of photonic integrated circuits and fluidic devices, mainly
due to their increasing performance, rapid processibility, capability
for precise tailoring of their optical properties, and their compara-
tively low cost. Another important aspect is the biocompatibility of
polymer materials and the fact that these materials are already in use
in many bio- and nonbio laboratories, which increases the acceptance
of polymer based optofluidic devices. This advantage requires the
improvement of fabrication technologies as well as the development
of application-specific tailored materials. As stated before, polymer
optical waveguides have been fabricated by various techniques, such
as dry etching, UV curing, and soft lithography replica-molding, and
embossing.
In recent years, hot embossing of microcomponents has become a
routinely used replication technology for thermoplastic polymers.
Low flow rates and slow molding speeds ensure that even the small-
est details in the nanometer range are replicated perfectly. Hot
embossing is particularly suited for structuring planar plates and
foils, as only a small amount of plastic has to be molded. In contrast
to injection-molding, the polymer flows a very short distance from
the foil into the microstructure during hot embossing. As a result,
very little stress is induced into the polymer and the molded parts are
well suited to optical applications, such as waveguides and lenses [8].
The setup of the hot embossing machine is relatively simple. Setup
times are short as the mold-insert and the polymer are easily
exchanged. Nickel shims of only a few hundred micrometers can be
used for replication without major effort. The electroplating process
for such shims takes much less time than for more compact tools, as
the electroplating time increases linearly with shim thickness. There-
fore, tools can be manufactured from an existing photomask design
within several days. A photograph of a nickel tool and an embossed
substrate is shown in Fig. 12-5.
In order to integrate optics and fluidics, it would be advantageous
to use a similar technology to create an optofluidic device. The deep
UV technique [9] is one method of combining optics, which includes
waveguides and light sources [10] and fluidics.
Two types of polymers have been investigated: PMMA (Hesa@
Glas, a homopolymer from Notz-Plastic, Switzerland) and alicyclic
methacrylate copolymers which were obtained from Hitachi Chemical