Medical Applications of Micro-Electro-Mechanical Systems (MEMS) Technology - Biomaterials Science: An Integrated Clinical and Engineering Approach - page 49

nearly perfectly vertical sidewalls. Also, the penetration depth of the X-ray
radiation into the PMMA layer is quite deep and allows exposure through
very thick PMMA layers, up to and exceeding 1 mm. After the development,
the patterned PMMA acts as a polymer mold and is placed into an elec-
troplating bath and nickel is plated into the open areas of the PMMA. The
PMMA is then removed, thereby leaving the metallic microstructure.
Because LIGA requires a special mask and a synchrotron (X-ray) radia-
tion source for the exposure, the cost of this process is relatively expensive.
A variation of the process that reduces the cost of the micromachined parts
made with this process is to reuse the fabricated metal part (step 5) as a
tool insert to imprint the shape of the tool into a polymer layer (step 3) (see

Cast PMMA
on metal
base

X-ray
Radiation

Separate
metal from
PMMA

1

5 2

Electroplate
through
PMMA

Develop
PMMA

4

3

FIGuRE 3.14
The steps involved in the LIGA process to fabricate high-aspect-ratio MEMS devices.

1780 µm

FIGuRE 3.15
SEM of a tall high-aspect-ratio gear made using LIGA technology.