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38 MEMS and Microstructures in Aerospace Applications

. Lithographie, Galvanoformung, Abformung (LIGA)
. Bulk micromachining
. Sacrificial surface micromachining

Figure 3.1 shows the basic concepts of each fabrication category. Bulk microma-
chining and sacrificial surface micromachining are frequently silicon based and are
generally very synergistic to the microelectronics industry since they tend to use
common tool sets.
Bulk micromachining utilizes wet- or dry-etch processes to produce an isotropic
or anisotropic etch profile in a material. Bulk micromachining can create large
MEMS structures (tens of microns to millimeters thick) that can be used for
applications such as inertial sensing or fluid flow channels. Commercial appli-
cations of bulk micromachining have been available since the 1970s. These
applications include pressure sensors, inertial sensors, and ink-jet nozzles.
Sacrificial surface micromachining (SSM) is a direct outgrowth of the processes
of the microelectronic industry and the materials used are largely silicon based. This
technology has had several commercial successes in the last decade, including in
optical mirror arrays and inertial sensors. Both these applications include integrated
microelectronics for sensing and control functions. This technology is generally
limited to film thicknesses of 2–6 mm; however, the resulting devices are assembled
as fabricated. This gives SSM technology a significant advantage for applications
involving large arrays of devices. Also, SSM technology has a path toward inte-
gration of electronics with the MEMS structures that will allow for control or
sensing applications.
LIGA technology was demonstrated in the 1980s. This technology can fabricate
devices with small critical dimension and high aspect ratio (i.e., thickness or width)
from metallic materials that can be electroplated. This provides advantages in
applications requiring a broad set of materials. However, assembly of large numbers
or arrays of devices is an issue.

3.3 LIGA
The LIGA process 13 is capable of making complex structures of electroplatable
metals with very high aspect ratios and thicknesses of several hundred microns.
The LIGA process utilizes x-ray lithography, thick resist layers, and electroplated
metals to form complex structures. Since x-ray synchrotron radiation is used as
the exposure source for LIGA, the mask substrate is made of materials transparent
to x-rays (e.g., silicon nitride, polysilicon). An appropriate mask-patterned layer
would be a high atomic weight material (e.g., gold).
The LIGA fabrication sequence shown schematically in Figure 3.2 starts with
the deposition of a sacrificial material such as polyimide, which is used for
separating the LIGA part from the substrate after fabrication. The
sacrificial material should have good adhesion to the substrate yet be readily
removed when desired. A thin seed layer of material is then deposited, which

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