substrate is transferred into a desiccator with 5μL of Triethoxy(1H,1H,2H,2H-
perfluoro-1-octyl)silane. The desiccator is pumped and the master is incubated
under vacuum for 4 h. Self-assembly layer of silane is formed on the surface during
that process. Next, the master is baked for 1 h to eliminate water and to cross-link
the ethoxy-groups thus stabilizing the coating.
The second step is, preparation of mixture of standard 10:1 (w/w) ratio of
elastomer and curing agent (Sylgard 184, Dow Corning). PDMS elastomer is
very viscous and it adheres to the walls of the beaker, so it is weighted first and
in excess. Then the corresponding amount of curing agent is added to that same
beaker using a Pasteur pipette. The two components need to be well mixed for a
successful polymerisation. Inevitably, bubbles will form while mixing and because
of the high viscosity they will remain in the mixture.
To remove the air entrapped in the mixture and to improve the reproducibility
between mouldings the mixture is placed inside a desiccator and the chamber is
then evacuated. Depending on the amount of mixture the degassing time can vary;
the rule of thumb is to keep the degassing until the foaming stops. Typically for 11 g
of mixture in 50 mL plastic beaker it would take 4–5 min of degassing. While the
mixture is inside the desiccator, the silanized master is placed in a petri dish.
Pouring the degassed mixture onto the master inside the petri dish is the
moulding step. Specifically for complex structures air may be trapped onto the
master; to avoid it a second degassing step is performed. Once the foaming stops the
moulded structure can be placed in an oven for curing at 85C for 2.5 h. The final
step of the casting process is cutting out the structure from the bulk of the mould and
consecutive de-moulding,i.e., detaching the PDMS slap from the master. Holes for
connectors are punctured at this stage and the PDMS cast is ready for bonding. In
my experience the best way to protect the master from dust or damage is to pour a
thin layer of PDMS on top and allow it to cure. This may look like waste of
elastomer but actually preserves your masters for future casting and moulding.
This is a protocol for the thermal curing of PDMS (Sylgard 184), other types of
PDMS chemistry exists that might be more suitable [ 10 ] for the functionality you
need. The authors report on hard PDMS with tensile modulus of 8.2 MPa in
comparison to 1.8 MPa for the 184-PDMS from Sylgard.
2.3 Hot Embossing
The hot embossing is a rapid fabrication process for thermoplastic material, which
is heated above a glass transition temperature, while being pressed against a master.
There are three parameters essential for optimisation of hot embossing: the first is
holding time, the second temperature on each platen, the third is embossing
pressure. We are going to touch upon each of the three in this section. The
embossing temperature should exceed by 15–40C the glass transition temperature
(Tg) of the polymer (Fig.3.5). Using a higher temperature leads to lower viscosity
of the polymer which facilitates filling of the features on the master and high fidelity
92 N. Dimov