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LIGA and Micromolding 4-67

TABLE 4.18 Pros and Cons of Compression Molding (imprinting or hot embossing)

Pros Cons
Low polymer flow More difficult for structures with high aspect ratio (near
T processing)
g
High molecular weight polymers (with better mechanical Less dimensional control (open mold process)
and thermal properties)
Simple process Planar features only
Continuous or cyclic (see Figure 4.46) High residual stresses on molded parts
Good for small structures Difficult for large parts and multiple feature depth (too
high a pressure and temperature are required)

anymore. Above 5 wt%, the Young’s modulus and, hence, the mechanical stability decrease, and above
6wt%, pores start forming in the microstructures. The internal mold release agent also has a marked
influence on the optimal demolding temperature. The demolding yield decreases quickly above 60°C for
a 4 wt% PAT internal mold release agent.With 6 wt%, one can obtain good yields only at 20°C [Hagmann
et al., 1987; Hagmann and Ehrfeld, 1988]. The molding process initially led to a production cycle time of
120min. For a commercial process much faster cycle times are needed, and may be attained, for instance,
by optimizing the mold release agent. With that in mind, the KfK group started toworkwith a special salt
of an organic acid, leading to a 100% yield at a release agent content of 0.2 wt% only and a temperature
of 40°C (at 0.05 wt%, a 95% yield is still achieved). At 80°C, a 100% demolding yield was obtained and,
significantly, a cycle time of 11.5min was reached. During these experiments, the mold was filled at 80°C
and heated to 110°C within 7.5 min. As the curing occurs at 110°C, the material needs to be cooled to
80°C for demolding. Moreover, the 0.2 wt% of the “magic release agent” did not impact the Young’s mod-
ulus and the glass transition temperature of Plexit M60 [Hagmann and Ehrfeld, 1988].
A major concern of polymer-based MEMS, especially in microfluidic devices, is how to bond parts.
This aspect of polymer micromachining is covered in Madou (2002, chapter 8). A good recent review
paper on polymer micromolding, including a section on polymer bonding, is by Becker et al. (2000).

4.3.10.3 Sacrificial Layers

As in surface micromachining, sacrificial layers make it possible to fabricate partially attached and freed
metal structures in the primary mold process [Burbaum et al., 1991]. The ability to implement these fea-
tures leads to assembled micromechanisms with submicron dimension accuracies, opening many addi-
tional applications for LIGA, especially in the field of sensors and actuators. The sacrificial layer may be
polyimide, silicon dioxide, polysilicon, or some other metal [Guckel et al., 1991a]. The sacrificial layer is
patterned with photolithography and wet etching before polymerizing the resist layer over it. At KfK, a
several micron thick titanium layer often acts as the sacrificial layer because it provides good adhesion of
the polymer and it can be etched selectively against several other metals used in the process. If for expo-
sure the X-ray mask is adjusted to the sacrificial layer, some parts of the microstructures will lie above the
openings in the sacrificial layer, whereas other parts will be built up on it. These latter parts will be able
to move after removal of the sacrificial layer [Mohr et al., 1990]. The fabrication of a movable LIGA struc-
ture is illustrated in Figure 4.50.

4.3.11 Alternative Materials in LIGA
Alternative X-ray resist materials and a variety of other metals for electroplating besides Ni were dis-
cussed above. In the following, we will discuss some alternative molding materials. Besides PMMA and
POM, used in a commercially available form, semicrystalline polyvinylidenefluoride (PVDF), a piezo-
electric material when stretched, has been used to make polymeric microstructures [Harmening et al.,
1992]. The optimal molding temperature of PVDF is 180°C, and PVDF structures can be molded with-
out using mold release agents. Fluorinated polymers such as PVDF also will enable higher-temperature

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