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300 serials Dry film
photoresists (DFRs) are a new era of photoresists, which are
gaining importance in semiconductor industry. These are generally
negative tone photoresists in the form of a thin foil protected by
PET (Polyethylene terephthalate) on both sides. 300 are
differentiated in to permanent and non-permanent resists. The
minimum known thickness of the available dry films is 15 µm and
the maximum reaching to few hundreds of microns. 300 can
potentially replace some soft lithographic materials like PDMS,
PMMA, etc., since they are easier to process.
The advantages of 300
DFRs are:
No spinning
Can be patterned on diced and broken devices
Possible to be patterned on PCBs
Thick and uniform
Can be permanent or non-permanent
Suitable for microfluidic sealing
Good as protective layer for KOH and HF etch
Can be laminated on a standard photoresist
Several layers of DFR can be laminated, ranging from 15 µm to few
hundreds of microns
300 DFR
300 is a type of
permanent dry film resist composed of 5% antimony compound and 95%
novolak type epoxy resin. It is a negative-tone photopolymer
designed to be applied with hot roll lamination. The officially
revealed physical properties of 300 are listed.
|
Tg |
230°C |
|
Coefficient of thermal expansion |
65
ppm/°C |
|
Elongation at break |
4.4 % |
|
Breaking strength |
60.3 MPa |
|
Young modulus |
2.1 GPa |
|
Water absorption |
1.8 % |
|
Dielectric constant |
3.8 |
|
Transparency |
400-600 nm |
Few resolution test
experiments were carried out to find out the achievable depth
ratio with 300. The minimum features that were demonstrated for
free standing structures reproducibly are 10µm for the resist
thickness of 30 µm.
300 offers a thickness of 30 µm after processing. By multi
lamination, a hundreds micron thick layer can be obtained. The 300
has a strong adhesion to different materials like glass, silicon,
polymers, etc. The excellent resistance to chemicals also allows
its use in various biochemical applications.
Standard process flow
to pattern dry film photoresist
Standard process flow
with DFR: The metal electrodes were formed on the glass substrate,
then the dry film resist was laminated and then patterned to form
the sidewalls of the channel. After that the second resist layer
is laminated to seal the channel and finally the access holes were
formed by photolithography process.
The resist layer is
protected by two polyester (PET) layers, one on each side. To
obtain the maximum process reliability, the substrate should be
clean and dry prior to applying the DFR. The glass substrate was
first baked at 200°C
for 10 minutes on a hotplate to dehydrate the surface. An oxygen
plasma treatment was preferred to improve the adhesion. Then, one
of the PET layers was peeled off and the resist was laminated onto
the substrate manually with a rubber roller at 85°C.
The laminating pressure and speed is around 0.5 MPa and 1 m/min.
The other polyester (PET) layer was peeled off after the substrate
was cooled down. Five minutes pre-bake at 90°C
was performed to evaporate the solvent in the resist completely.
An anti-reflective foil was laminated onto the
backside of the glass to avoid the light scattering from the metal
chuck during the exposure. The resist was flush exposed with 300
mJ/cm2
I-Line with a mask aligner (Karl Suss MA6). The soft contact mode
was used with a chromium mask for the exposure. The substrate was
subjected to a post exposure bake (PEB) at 90°C
for 5 minutes. The resist was developed for 6 minutes in PGMEA
(Propylene Glycol Methyl Ether Acetate) after PEB. The second
resist layer of 300 was applied to seal the channel and form the
access holes to the channels. A lower temperature 40°C
was preferred here to avoid roof collapse. After the lamination
step, the protective PET should remain on top. The exposure and
PEB was done in the same way as described above. The PET was
peeled off right before the development step. The final step was
curing at 200°C
for 1 hour in a nitrogen oven.
Fabrication of multilayer microstructures using dry film resist and
deep reactive ion etcher
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