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and far away from the pillar (7.47µm at 40µm from the left
side pillar).
The flow of the resist along the pillar during lamination is
the major difficulty we have encountered during the
development of this process. In order to avoid this problem,
we have optimized process parameters. Figure 8 shows an
example of flowing effect. This problem becomes really
significant when microstructures have low dimensions as it
reduces drastically the volume of encapsulated fluid. In our
case it has been considered negligible regarding the final
volume of each structure (1800µl). However, this
phenomenon has been reduced by increasing the soft bake
time (near twice the initial value) and by lowering the
pressure and speed lamination to the minimum value
allowed by the laminator. It has been tried to increase the
thickness of the plastic layer which supports the resist but
no improvement has been observed.
11-13
May 2011, Aix-en-Provence, France
2 – Liquid filling and sealing 3 – Removal of the top PET film
Fig. 9. Realization of the second level.
The figure 10 illustrates a SEM view of a quite complete
two-level of microstructures showing tanks and pillars of
the first level, IL covering the all first surface, and pillars of
the second level.
Pillar of the 2 nd level
Upper tank level
Interface layer
lower tank level
Pillar of the 1 st level
Fig. 10. SEM view of empty structures without the final cover.
Fig. 8. Example of a resist flow after the lamination (in the circle) before
the optimization of the process.
The first level of microstructures has been filled and
sealed so the fabrication of the second level of
microstructure can begin.
D. Fourth step: fabrication of the second level and its
sealing.
In order to realize the second level of microstructures we
have to repeat the same operations as described before (Fig.
9). The first sealed level is treated with an oxygen plasma
(P=200W, t=60s, V=200ml) in order to increase the
wettability. The water drop angle is equal to 70° before
plasma O 2 and falls down to 8° after treatment. The plasma
O 2 increases also the adhesion of the spin coated resist on
the IL surface of the first level. Then, the photo resist is
spin-coated on the IL and soft backed as already before.
Then using an alignment process on the Canon stepper, the
second level is UV exposed. After a post exposure bake the
second level is revealed. All parameters of these operations
are the same those used in the fabrication of the first level.
The second level has to be processed in a short delay (less
than 2 hours) in order to avoid stress issues inducing a
deformation of structures. The curvatures over the tanks can
be increased from 300 nm to six microns if the time
between the fabrication of level 1 and 2 is greater than 12
hours.
1- UV exposure and SB
III. CONCLUSION
We have developed a fabrication process to realize multilevel
structures with encapsulated liquid. A chessboard
structures have been chosen for demonstration but more
complex structures could be realized as each level could be
patterned. We stacked up two levels of reservoirs, but there
is no limitation to go further and stack up other layers
following the same process. The total area of the structure is
118 cm² but it could be extended to larger area. We
observed no formation of bubbles or liquid accumulation
over pillars. This technology offers the possibility to easily
implement optical functions at low cost and on large area
and is therefore a basic step towards optofluidics
applications.
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