If it is manually dispensed from a small bottle it is recommended, to minimize the
distance between the bottle and the substrate surface. Second, the wafer is spun
(e.g., 3000 RPM) to create uniform coating. Third, is soft bake (5 min) at 65C,
which is followed by (2 min) incubation at 95C. Fourth comes the alignment and
exposure using hard contact with vacuum for better resolution. A delay before
exposure is optionally introduced that lasts from 4 min to 2 hours and serves as
stress reducing step in the photoresist. The total exposure time for this particular
case (SU-8 25, spun at 3000 RPM) is 60 s on 12 mW/cm^2 EVG mask aligner. Fifth
step is post exposure bake, (5 min) at 65C followed by 95C (2 min). Finally the
latent image is developed in REP 600 (5 min) with agitation. The wafer is rinsed
with isopropanol and after drying is ready to use. The time values provided in
brackets may change as per the datasheet provided by the supplier; depending on
the fabrication conditions.
Patterning of positive photoresist (i.e., OiR-, AZ-series or PMMA-based) fol-
lows similar processing steps: spin of photoresist; soft bake to remove the solvent
from the resulting layer; expose the wafer and resist using UV-light; develop the
resulting pattern. To promote adhesion the substrate is treated with HDMS. But the
major difference is the polarity of the used mask as after exposure the illuminated
areas become soluble. In terms of application, the positive photoresists are not
building the channel walls but serve for masking during etching or spattering
followed by lift-off processes.
2.2 Moulding and Casting
The principle of moulding is that a master, bearing an inverse image of the targeted
structure, is covered with a liquid polymer, or other material that solidifies after
exposure to light, temperature or other external factors. As the resin polymerises it
mimics the features of the master. At the end of the process the replica is demoulded
leaving the master intact, and ready-to-use, which makes the whole process appli-
cable for rapid device generation.
The most encountered moulding material is polydymethylsiloxane PDMS,
which is heavily brunched elastomer, that gains its popularity among researchers
due to its multiple implementations in cell studies. The properties of PDMS that
potentially have adverse effects for microscale cell studies are discussed in a critical
review [ 8 ] covering the following material characteristics: deformation, evapora-
tion, absorption, leaching of uncrosslinked oligomers, and hydrophobic recovery. It
has Young’s modulus ranging from 8.7 to 3.6 kPa [ 9 ] that is tailored by varying the
ratio between the elastomer and crosslinking agent. In this chapter the process flow
of PDMS moulding onto SU-8/silicon master is conveyed.
Surface functionalization of the mould is the first step in this process. In the case
of silicon substrate a straightforward approach is to use vapour deposition of
fluorinated silane to render the surface of the master hydrophobic. For this purpose,
the surface of the master is activated using 30 W RF plasma for 1 min. Then the
3 Manufacturing Methods Overview for Rapid Prototyping 91