Microfluidics for Biologists Fundamentals and Applications

of replication. However, the polymer might degrade if the temperature is too high.
Ideally, the temperature of the tool and substrate should be the same within a range
of 2 C during the holding time. Increasing the holding time gives the polymer
more time to flow into the mould resulting in better replication. Also more complex
architectures from the master require longer holding times allowing for the molten
plastic to fill the features. A higher pressure obviously pushes the polymer faster in
to the mould but the disadvantage is that it also increases the risk of damaging the
master, especially when using SU-8 on Si wafer. Breaking a master takes you
back to photolithography and potentially into the cleanroom, which is time and
cost intensive. The parameters are interconnected which makes optimisation
nontrivial task.
A conservative approach to optimize the hot embossing consists of isolating a
single parameter by keeping the remaining two constant. For instance, hot
embossing of PMMA, having glass-transition temperature of 105C, will require
heating to 130C and pressure of 290 N/cm^2 with varying holding times that
depend on the complexity of the featured design. Studies exist that develop meth-
odologies for process optimisation [ 11 ]. The more advanced optimisation methods
include deterministic studies based on numerical simulations and computational
fluid dynamic (CFD), which aim to elucidate critical values [ 12 ]. In practice the
process optimisation targets a specific outcome. For instance to increase the
lifespan of the master; first, holding time, then embossing temperature and finally
pressure are altered. Such optimisation approach will increase the number of
replications per master by reducing the risk of damage, but might also result in
prolonged cycle times. In contrast, time efficacy is achieved through changes in
embossing pressure, and only then holding time.

Displacement vs Temperature

2

1.5

1

Position(mm)0.5

0

-0.5

80 100

T<<Tg

T~Tg

T>Tg

T>>Tg

120 140

Temperature (Celsius)

160 180 200

a b

Fig. 3.5 Jenoptik HEX02 hot embossing machine, vacuum chamber and mould insert fixture
(left). Displacement of the platens as a function of temperature with different states of the polymer
material (right). Ideal temperatures for bonding is about T>Tg but it can vary depending on the
design complexity. Reprinted with the permission of [Springer-Verlag, 2006] [ 11 ]

3 Manufacturing Methods Overview for Rapid Prototyping 93

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