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coefficient compatible with the microstructure material. The
photo resist used for the microstructures is a negative one
epoxy based. After the polymerization, the photo resist is
rigid enough to be used as a cover. In this way there is no
mechanical incompatibility because the same material has
been used for the microstructures and the fabrication of the
cover.
This 3 rd step is divided in two parts: the first one is the
preparation of the cover which is called in this document the
interface layer (IL) and the second one is the lamination of
this IL onto the first level of the chessboard structure. The
figure 5 illustrates the different operations realized for this
step.
In a first time, the cover is built. First of all, a PET film
coated with a layer of glue (a weakly adhesive one) is
laminated onto a thick glass wafer. Then the photo resist is
spun-coated: it will be the IL. The thickness of the IL is set
to 7 µm. The photo resist is soft baked at 50°C during 25
minutes and then a room temperature stabilization delay is
applied (at least 20 minutes long) (Fig. 5-1). We will take
advantage that the photo-resist is still sticky after the SB to
perform an efficient lamination onto the first level
chessboard. The plastic film with the IL will be removed
from the substrate (Fig. 5-2) just before performing the
lamination onto the first level chessboard for the sealing.
In a second time, the surface of the microstructures of the
first level is activated by oxygen plasma (P=400W, t=30s,
V=400ml). A drop of liquid is dripped down on the
structure at the location where the lamination begins. In that
way the liquid is pushed in the structure all along the
lamination. This technique is interesting when only one type
of liquid has to be filled in the tanks. At the end of the
lamination, a full wafer UV exposition is done, associated
to a PEB (still we the plastic film) at a low temperature
(50°C) in order to fully polymerize the IL. The UV
exposition has to be made quickly after the lamination in
order to limit the solvent diffusion in the non polymerized
resist at this time. After the PEB and a room temperature
stabilization time, the plastic film is removed. Stages of this
process step are illustrated by Fig. 5.
1- Fabrication of
IL on PET film
Substrat
11-13
May 2011, Aix-en-Provence, France
Resist (7µm)
PET
Paste
4- Removal of the PET film
Fig. 5. Schematic representation of the filling and sealing step.
The chessboard structure used in this experiment has
sides of 300µm. The flatness of the IL (top side) has been
measured after the lamination with a optical profilometer.
The experimental profile variation is less than 400 nm high
on 300 µm long as illustrated in figure 6.
b)
a)
c)
Fig. 6. Interferometric measurement over 1,2 x 0,9 mm area (a), horizontal
profile line (b) and vertical one (c) of the cover of the level one after filling
and sealing.
In Fig. 6, it can be observed that the cover is shaped as a
bump above tanks. A bump down was expected due to
pressure applied during the lamination. However the
amplitude of the deformation is low (350nm) and is not
visible in the cross sectional view of figure 7
2- Removal of PET
film with the IL
3- Lamination of the
PET film onto the
chessboard structure of
the first level
Fig. 7. SEM Cross-section view of tank with pillar and the cover (left) optical
top view of filled and sealed strucrure (right).
Fig. 7 shows also that the resist flew in the tank. It is
visible on the right side of the pillar. There, the cover
thickness is 9.37 µm (SEM measurement). Anywhere else,
it is close to the expected value i.e 7 µm (physically
measured with a mechanical profilometer). This is
confirmed by the measurement above the pillar (6.94 µm)
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