research activities in 2007 - CSEM

Bonding of Glass or Silicon Chips with a Self-Sealing Photostructurable Elastomer
J. Auerswald, F. Cardot, P. Niedermann, A. Ibzazene, M. Fretz, N. Schmid, H. F. Knapp
When it comes to the integration of planar electrodes on glass, quartz, silicon or thermoplastic chips into microfluidic systems, standard bonding
methods like diffusion bonding, anodic bonding, thermo-compression bonding etc. do not work. Surfaces carrying electrodes cannot be sealed with
a stiff material. The challenge is even bigger when two glass chips with facing electrodes and microfluidic channels in between are required by the
application, with an alignment precision down to a few micrometers. Photostructurable polysiloxane could be a solution.
The use of photostructurable polysiloxane combines two
advantages: Firstly, the good alignment precision of
microfluidic channels made by photolithography. Secondly,
the reliable permanent bond of silicones to glass, even if the
glass chips carry electrodes. Photostructurable polysiloxane is
a material known from ISFET and ChemFET sensor
packaging. There, typical lateral structure dimensions are
several millimeters, sometimes slightly below 1 mm, and are
shaped as simple O-rings [1, 2] . However, the use of this
material class for microfluidic systems with microfluidic
channel networks containing junctions or intersections at
channel widths well below 1 mm, and electrodes in the
channels has not been demonstrated yet. One of the critical
issues is the precise bonding of the cured material to the glass
counter chip which also has electrodes on its surface.
Figure 1: LEFT: Bonded glass chips with facing top and bottom
planar electrodes, alignment marks and fluidic access holes. The
microfluidic channels are photo-structured into the elastomeric
intermediate layer. RIGHT: Bonded chip filled with a colored fluid.
25 μm high microfluidic channels with a width down to 200 μm
(and less), with channel junctions, into photostructured
polysiloxane on a glass substrate with planar electrodes were
structured. It was further achieved to permanently bond the
cured material to counter glass chips with planar electrodes.
The whole microfluidic system was leak tight, even without an
external clamp (Figure 1). The elastomer-based
photolithography and the precise bonding process allow for
alignment accuracies down to a few micrometers.
This high precision combined with good sealing provides a
viable alternative to the until now rather unsatisfying sealing
results obtained with classic negative tone photoresists such
as SU-8, BCB, or polyimide.
A possible application is shown in Figure 2. First,
microelectrodes are fabricated on a bottom glass wafer. Then,
in a second mask process, the photostructurable polysiloxane
is deposited and structured. Before dicing, the bottom wafer is
protected by a removable resist layer. After removal of the
protective resist and surface activation, the chips can be
bonded to the diced chips of the top wafer. The top glass
chips also have electrodes and pre-drilled fluidic access holes.
The bond is permanent, due to the applied surface activation.
Fluidic connectors can be glued or tape-bonded above the
fluidic access holes.
Figure 2: CAD explosion model (left) and global view of the bonded
chip with facing planar top and bottom electrodes, and Luer
connectors (right).
The alignment precision depends on the used bonding
machine and can be as good as 1-2 micrometers with a
device bonder. The chips sealed well over the entire gage
pressure range up to 28’000 Pa (4 psi), even after the 10th
pressure cycle (Figure 3). This is more than enough for typical
microfluidic applications.
Figure 3: Example of a sealing test. The chips seal well at pump
gage pressures of 30’000 Pa (4 psi), even after the 10 th pressure
cycle. This is more than good enough for typical microfluidic
applications.
Potential applications include high-end niche markets for labon-chip
systems designed for dielectrophoresis, dielectric
spectroscopy, low voltage AC-EOF pumping and other
microfluidic applications requiring photolithographic
electrodes.
This work was partly funded by the EU (project IST-FP6-
027540, IntegramPLUS). CSEM thanks them for their support.
[1] P. Temple-Boyer, J. Launay, I. Humenyuk, T. Do Conto,
A. Martinez, C. Beriet, A. Grisel, Microelectronics Reliability 44
(2004) 443-447.
[2] P. Arquint, M. Koudelka-Hep, B.H. van der Schoot, P. van der
Val, N. F. de Rooij, Clin. Chem. 40/9 (1994) 1805-1809.
95