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4-70 MEMS: Design and Fabrication

FIGURE 4.51 Large aspect ratio magnetic motor. (Courtesy of IMM, Germany.)

engineering can be complementary techniques for producing individual parts that have to be assembled
afterward. Only components with the smallest features are produced by means of microfabrication tech-
niques; other parts are produced by traditional precision mechanical methods.
The University of Wisconsin team [Guckel et al., 1991b] has also demonstrated a magnetic LIGA drive
consisting of a plated nickel rotor that is free to rotate about a fixed shaft. Large plated poles (stators)
come close to the rotor. The rotor is turned by spinning a magnet beneath the substrate. Further work by
the same group involved enveloping coils, used to convert current to magnetic field in situ rather than
externally [Guckel et al., 1992]. It is necessary to plate two different materials — a high-permeability
material such as nickel and a good conductor such as gold — to accomplish this.
The motor also can be used as electrostatic motor. By applying voltages to the stator arms, the rotor
(which is grounded through the post about which it rotates) is electrostatically attracted and can be made
to turn by poling the voltages. Wallrabe et al. (Wallrabe et al., 1994) present design rules and tests of elec-
trostatic LIGA micromotors. Minimum driving voltages needed were measured to be about 60V; opti-
mized design torques of the order of some mNm are expected. The cost of electromagnetic LIGA
micromotors will often urge the investigation of LIGA-like technologies or hybrid approaches (LIGA
combined with traditional machining) as more accessible and adequate machining alternatives.

Example 2. LIGA Spinneret Nozzles
Profiled capillaries (nozzles) in a spinneret plate for spinning synthetic fibers from a molten or dis-
solved polymer, as shown in Figure 4.52, normally are produced by micro-EDM. This process establishes
a practical lower limit of 20 to 50 µm for the minimal characteristic dimension, and the method cannot
satisfy the requirements to produce complexly shaped, high-aspect-ratio spinnerets at a low cost. Smaller,
more precise, and high-aspect-ratio nozzles can be produced by LIGA [Maner et al., 1987].
Spinneret nozzles lend themselves to LIGA from the technical point of view. Compared with fabrica-
tion by micro-EDM, the minimum characteristic dimensions with LIGA can be reduced by an order of
magnitude, and a high capillary length with excellent surface finish can be obtained easily. Moreover, the
LIGA process makes all the nozzles in parallel, while micro-EDM is a serial technique. The market might
focus on niche applications such as specialty, multilumen catheters. Also, medical use of atomizers for
dispensing drugs is projected to increase dramatically in the coming years; this might create a demand for
awide variety of precise, inexpensive micronozzles.
Shew et al. used a conformal mask to fabricate a LIGA die with capillaries 2mm deep and 70 µmwide
for the mass production of electroplated spinnerets for the spinning of polyester fibers [Shew et al., 1999].
The conformal mask eliminates the need for the alignment steps accompanying the multiple exposure
processes required in the deep X-ray lithography used by these authors.As illustrated in Figure 4.53a, a thin
sputtered copper layer covers a thick PMMA layer on an Al substrate [a]. Aresist layer (JSR 137N, Japan)
is then patterned on the copper layer by UV lithography, followed by plating Au absorbers [b]. After strip-
ping the resist and the copper layer [c], the conformal mask is ready for multiple X-ray exposure and

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