Online proceedings - EDA Publishing Association

11-13
May 2011, Aix-en-Provence, France
In a second approach, we used another PMMA plate as a
lid substrate. DFR was laminated on this plate and brought a)
into contact with the channel layer. Defined pressure and
temperature were applied by passing this stack through the
desktop laminator again, so that bonding took place. Final
polymerization and bond stabilization was attained by
exposing the DFR to UV-light through the transparent
PMMA lid plate after bonding.
Akin to this, we also used a PMMA lid substrate for the
third bonding technique. Instead of DFR, a 10 µm SU-8
layer was deposited on this lid plate. This adhesive layer
was laid on the channel structure and bonded under certain
pressure and temperature conditions. Again, exposure was
carried out through the PMMA lid substrate after bonding.
A related approach was attempted in [5], but using a predrilled
lid substrate, which led to deposition and bonding
problems as will be described below.
III.
RESULTS AND DISCUSSION
A. Channel Layer Fabrication
Both SU-8 and DFR were found to be suitable as channel
layer materials as clearly defined channels with straight
walls were fabricated. However, DFR allows aspect ratios
up to approximately 2:1, whereas SU-8 can be used for any
channel dimensions up to more than 10:1. Fig. 3 a) shows a
confocal microscope picture of channels with widths
between 1.34 µm and 12 µm and a height of 20 µm. Due to
the weaker optical properties and surface quality of PMMA
compared to glass, scattering of the exposure light was
expected, which would lead to difficulties in producing high
aspect ratios. However this problem held off and even
channels with an aspect ratio higher than 10:1 were
fabricated. Furthermore, the use of PMMA as substrate
material is advantageous compared with the commonly used
silicon or glass due to its coefficient of thermal expansion
(CTE). As mentioned in [5], the CTEs of PMMA and SU-8
(85 ppm/K and 52 ppm/K, respectively) are closer to each
other than SU-8 and silicon (2 ppm/K) or borosilicate glass
(3,25 ppm/K). Similar thermal expansion coefficients
prevent distortion of resist during the baking steps.
Moreover, no flaking of resist from PMMA or from the
adhesive resist layer was visible, not even near the drilled
holes (Fig. 3 b)). Thus, good adhesion of the layers was
provided, although there were no elaborate cleaning steps
necessary prior to deposition but only flushing of the
PMMA plate with IPA and nitrogen.
B. Fluidic Interfaces
Mechanical drilling of the fluidic inlets and outlets
proofed of value as a very simple and efficient technique.
By means of the protective foil, absolutely no damaging of
the channel layer occurred and the foil was removed
without difficulties. The drilled holes were well defined
without chipping and no cracks or other damages were
induced (see Fig. 3 b)).
In Ref. [5], fluidic inlets and outlets were drilled into the
PMMA lid plate instead of the PMMA substrate which
contains the channel structures. SU-8 was not suitable as an
b)
Fig. 3. a) Confocal microscope image of structured SU-8 on PMMA
(height: 20 µm). The black line marks the position of the profile shown
below. b) Confocal microscope image of a channel with a fluidic inlet
drilled into PMMA.
adhesive layer as it formed cords because of the holes.
Thus, DFR was deposited on top of this pre-drilled PMMA
lid to serve as the adhesive bonding layer. When the release
liner of the DFR was removed, most inlets and outlets were
open as the DFR layer could not stick to a surface in these
areas. In doing so, some pieces of DFR fell onto the
deposited adhesive layer where they formed artifacts and
led to bond defects. This problem, that also involves low
yield and reliability, is completely evaded using the new
fabrication process.
C. Closing of Channels
Comparing the adhesion bonding techniques, all three of
them turned out to be successful methods for closing of
channels. On account of exposing the resist after bonding,
good adhesion and bond strengths were assessed. Yet each
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