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40 MEMS and Microstructures in Aerospace Applications

PMMA
Seed Material Sacrificial Material
Substrate

a. Substrate with sacrificial material, seed material, and PMMA applied.

x-ray illumination
Mask

Exposed PMMA

b. Exposing PMMA with x-ray synchrotron radiation.

Electroplated metal

c. Electroplated metal in the developed PMMA mold.
FIGURE 3.2 LIGA fabrication sequence.

will enable the electroplating of the LIGA base material. A frequently used seed
material would be a sputter-deposited alloy of titanium and nickel. Then a thick
layer of the resist material, polymethylmethacrylate (PMMA), is applied. The
synchrotron provides a source of high-energy collimated x-ray radiation needed to
expose the thick layer of resist material. The exposure system of the mask and x-ray
synchrotron radiation can produce vertical sidewalls in the developed PMMA layer.
The next step is the electroplating of the base material (e.g., nickel) and polishing
of the top layer of the deposited base material. Then the PMMA and sacrificial
material are removed to produce a complete LIGA part.
Since LIGA can produce metal parts, magnetic actuation is feasible. Figure 3.3
shows an assembled LIGA mechanism. Assembly of LIGA devices for large-scale
manufacturing is a challenging issue.

3.4 BULK MICROMACHINING

Bulk micromachining uses wet- 14 and dry etching methods 15,16 to achieve isotropic
and anisotropic etches of features in materials. In order to manufacture items of

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