Page 328 -

LIGA and Micromolding 4-69

temperatures, the plastic mold and the organic slurry components are removed completely in a burnout
process at lower temperatures. First results have been obtained for zirconium oxide and aluminum oxide.
An important application for ceramic microstructures is the fabrication of arrays of piezoceramic
columns embedded in a plastic matrix. Because the performance of these actuators is linked to the height
and width ratio of the individual ceramic columns as well as to the distance between the columns in the
array, LIGA’s tremendous capacity for tall, dense, and high-aspect-ratio features makes it an ideal fabri-
cation tool [Preu et al., 1991; Lubitz, 1989].
An interesting method to make LIGA ceramic or glass structures in the future may be based on sol-gel
technology. This technique involves relatively low temperatures and may enable LIGA products such as glass
capillaries for gas chromatography (GC) or possibly even high-T superconductor actuators. Sol–gel tech-
c
niques should work well with LIGA-type molds if they are filled under a vacuum to eliminate trapped gas
bubbles.In the sol–gel technique,a solution is spun on a substrate that is then given an initial firing at around
200°C, driving off the solvent in the film. Subsequently, the substrate is given a high-temperature firing at
800 to 900°C to drive out the remaining solvents and crystallize the film. A major issue is the large shrinkage
of sol–gel films during the initial firing. For example, the maximum thickness of high-T superconductor
c
films currently achievable measures approximately 2µm due to the high stress in the film caused during
shrinkage. The latter is related to the ceramic yield or the amount of ceramic in the sol–gel compared with
the amount of solvent. The sol–gel technique in general suits the production of thicker films well, as long as
high-ceramic-yield sol–gels are used. As LIGA-style films tend to be thick, areduction in the amount of sol-
vents to allow a high ceramic yield after firing would be needed to make sol–gel compatible with LIGA. This
would require some development. Also, lead–zirconium–titanate (PZT) devices could be made with sol–gel
technology and LIGA, most likely by applying the metal-plated structure rather than the direct PMMA tem-
plate for the creation of the device, because PZT must be processed in several elevated temperature steps.
PZT sol–gel contains no particles that would prevent flow into small channels in a plated mold. The sol–gel
contains high-molecular-weight polymer chains that hold the constituent metal salts, which are further
processed into the final ceramic film. Thus, the sol–gel could be formed into the mold in a process much
like reaction–injection molding, with the mold being filled with the chemistry under vacuum.

4.4 Examples

Example 1. Electromagnetic Micromotor

LIGA makes better electrostatic actuators than other Si micromachining techniques; the same is true
for electromagnetic actuators. Most Si micromachined motors today produce negligible amounts of
torque. In practical situations, what is needed are actuators in the millimeter range delivering torques of
10 6 to 10 7 Nm. Such motors can be fabricated with classical precision engineering or a combination of
LIGA and precision engineering.
The performance with respect to torque and speed of atraditional miniature magnetic motor with a
1mm diameter to 2mm long permanent magnetic rotor was demonstrated in practice to be incompar-
ably better than the surface micromachined motors discussed in chapter 3 [Goemans et al., 1993]. The
small torque of the surface micromachined electrostatic motors is due largely to the fact that they are so
flat. The magnetic device made with conventional three-dimensional metalworking techniques has an
expected maximum shaft torque of 10 6 to 10 7 Nm. The torque depends on the Maxwell shearing stress
on the rotor surface integrated over the area of the latter; the longer magnetic rotor easily outstrips the
flat electrostatic one with the same rotor diameter [Goemans et al., 1993]. In both electric and magnetic
microactuators, force production is proportional to changes in stored energy. The amount of force that
may be generated per unit substrate area is proportional to the height of the actuator. Large-aspect-ratio
structures are therefore preferred.
With LIGA techniques, large-aspect-ratio magnetic motors as shown in Figure 4.51 can be built
[Ehrfeld et al., 1994]. Essentially, a soft magnetic rotor follows a rotating magnetic field produced by the
currents in the stator coils. The motor manufacture is an example of how micromachining and precision

© 2006 by Taylor & Francis Group, LLC