Page 194 - Microsensors, MEMS and Smart Devices - Gardner Varadhan and Awadelkarim

174 MICROSTEREOLITHOGRAPHY FOR MEMS

7.1.1 Photopolymerisation

SL is a photopolymerisation process, that is, a process that joins together a number of
small molecules (monomers) in a resin or resist to make larger molecules (polymers),
which usually use UV radiation to polymerise (or cure) the resist material. Different
types of photopolymers are used in SL prototyping and these are commonly based on
either free-radical photopolymerisation or cationic photopolymerisation. The generalised
molecular structure of monofunctional acrylate, epoxy, and vinyl ether, which are the
three main photopolymer systems, are shown in Figure 7.2 (Jacobs 1996).
In general, the photopolymerisation process is initiated by the incidence of photons
generated by an UV light source. The breaking of the C—C double bond (acrylate and vinyl
ether) or ring (epoxy) in the monomer enables monomer units to link up and form a chain-
like structure. The cross-linked polymer chain finally forms when the chain propagation
is terminated. To illustrate this process, the various steps in a free-radical polymerisa-
tion sequence are given in Figure 7.3. The selection of the photopolymer depends on the
required dimensional accuracy and mechanical properties of each individual photopolymer
formulation (Jacobs 1996).
The curing depth and line-width of the photopolymerisation process are the two most
critical parameters and these need to be carefully controlled in the SL process. In principle,
the curing depth and line-width can be determined from the beam distribution and the
absorption of radiation in the resist (Figure 7.4).

Elevator

VatofUV
curable
solution

Figure 7.1 Basic principle of stereolithography: the writing of 3-D patterns into a series of layers
of UV-curable resist at different heights

Acrylate
,0

A
Epoxy

Vinyl ether

Figure 7.2 Generalised molecular structure of three polymers used in MSL: acrylate, epoxy, and
vinyl ether. From Jacobs (1996)