(392h) Fabrication of 3D Electrodes for Electrorotation Experiments

Fabrication of
3D Electrodes for Electrorotation Experiments

Samuel
Kilchenmann*
and Carlotta Guiducci*

*École Polytechnique
Fédérale de Lausanne, SWITZERLAND

Introduction

Electrorotation
(ROT) is a powerful technique that has been used to characterize and
differentiate different particles through the extraction of their electrical
parameters. Through the study of a particle's response to an electric field at
specific frequencies, it is possible to extract information such as the
membrane integrity of biological particles. In order to achieve high accuracy
of the extracted parameters, a rotating electric field with high linearity needs
to be generated. For this purpose, Goater et al. introduced bone-shaped
electrodes [1]. Most devices
used for electrorotation consist of planar electrodes that result in electrical
fields in z-direction, which causes out of focus movements and makes trapping
and observation of the particles difficult. Different groups tried to overcome
this problem by designing chips with octopole cages that trap and rotate cells
at the same time [2] or by combining
the ROT devices with laser tweezers for trapping [3]. We introduce a
novel fabrication process for three-dimensional microelectrodes that can be
used for electrorotation experiments. The advantages of our design are that the
generated electric field does not have any z-component and thus no out-of-focal-plane
movements are exerted on the particle. Moreover, the electrodes serve as a kind
of mechanical trap for the particles. Further advantages of the presented fabrication
process are that the microfluidic structures can be aligned with
photolithographic precision and that it is possible to work with bulky and short-working
distance lenses.

Simulation

The
electrical fields that are generated by 3D and planar electrodes were simulated
using Comsol Multiphysics. In both cases a bone shaped design of the electrodes
was used for simulation. From the top views in Fig. 1 it can be seen that
both designs lead to highly linear fields in the x-y plane, however, in the
case of the planar electrodes the generated field lines have components in
z-direction as well. A further advantage of 3D electrodes is that local heating
effects can be reduced by about an order of magnitude, while keeping in the
center the same field strengths as with the planar electrodes. This is due to
the higher electrode surface that is used to apply the potential, which leads
to a reduction of the current density.

Microfabrication

The
microfabrication process is shown in Figure 2. We start with a silicon
wafer passivated by Si₃N₄ and SiO₂ layers with
thicknesses of 200 nm each. First, a photoresist is applied by spin
coating and is structured by photolithography. The oxide and the nitride layers
are etched and the design is transferred 300 μm deep into the silicon
by a Bosch process. Then, a 10 μm thick SU-8 layer is patterned by
photolithography. For better adhesion of the metal on the SU-8, the resist is
hardbaked at 135°C and plasma activation is done before sputtering a Ti/Pt
layer onto the structures. Afterwards, a dry film resist is laminated on the
wafer and patterned by photolithography. The resist serves as etch mask for
plasma etching of the metal layer. After stripping of the photoresist, a second
SU-8 layer to pattern the microfluidic channels is applied. To planarize the
SU-8 layer that is spun onto the high topographic surface, chemical mechanical
polishing (CMP) is performed after the post exposure bake and before the
development of the resist. This strategy is chosen that the undeveloped resist
protects the access pads during CMP. Afterwards, the SU-8 is developed and the
microfluidic accesses are opened up by etching the backside oxide and nitride
layers and by grinding the wafer down to 300 μm from the backside. Finally,
the wafer is diced and a coverslip is bonded onto the chips, where the SU-8
serves as adhesive. In Figure 3 we show a picture of such an
electrorotation chip (a) and SEM images of (b) the 3D electrodes without glass
coverage of the microfluidic channels and (c) the cross section of the whole
chip with glass coverage.

Figure
1 Comsol simulations of the electric field for planar and 3D electrodes

Figure
2 Fabrication process of the electrorotation chips with 3D electrodes

Figure
3: (a) Electrorotation chip ; (b)SEM of the 3D metal electrodes; (c)SEM of the
cross-section of a chip with the glass coverage.

1.         Dalton, C., et al., Parasite viability by
electrorotation. Colloids and Surfaces a-Physicochemical and Engineering
Aspects, 2001. 195(1-3): p. 263-268.

2.         Reichle, C., et al., Electro-rotation in
octopole micro cages. Journal of Physics D-Applied Physics, 1999. 32(16):
p. 2128-2135.

3.         Schnelle, T., et al., Combined
dielectrophoretic field cages and laser tweezers for electrorotation.
Applied Physics B-Lasers and Optics, 2000. 70(2): p. 267-274.