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62 MEMS and Microstructures in Aerospace Applications

Ge x an attractive micromachining material for monolithic integration with micro-
electronics, which requires a low thermal budget. 44
Also, a surface micromachining process can be implemented utilizing poly-
Si 1 x Ge x as the structural film, poly-Ge as the sacrificial film with a release etch of
hydrogen peroxide when x < 0.4. Poly-Ge can be deposited as a highly conformable
material that enables many MEMS structures.

3.7.3 DIAMOND
Diamond and hard amorphous carbon form a promising class of materials that have
extraordinary properties, which promote new applications for MEMS devices. The
various amorphous forms of carbon such as amorphous diamond (aD), tetrahedral
amorphous carbon (ta-C), and diamond-like carbon (DLC) have hardness and
elastic modulus properties that approach crystalline diamond, which has the highest
hardness (~100 GPa) and elastic modulus (~1100 GPa) of all materials. 45 The
appeal of this class of materials for MEMS designers is the extreme wear resistance,
hydrophobic surfaces (i.e., stiction resistance), and chemical inertness. Recent
progress has been achieved in the area of surface micromachining and mold-
based processes 46,47 and a number of diamond MEMS devices have been demon-
strated. 48,49 The use of diamond films in MEMS is still in the research stages.
Recent progress in stress relaxation of the diamond films 50,51 at 6008C has been
essential to the development of diamond as a MEMS material.

3.7.4 SU-8
EPON SU-8 (Shell Chemical) is a negative, thick, epoxy-photoplastic high aspect
52
ratio resist for lithography. This UV-sensitive resist can be spin coated in a
conventional spinner to thicknesses ranging from 1 to 300 mm. Up to 2-mm
thicknesses can be obtained with multilayer coatings. SU-8 has very suitable
mechanical and optical properties and chemical stability; however, it has the
disadvantages of adhesion selectivity, stress, and resists stripping. SU-8 adhesion
is good on silicon and gold, but on materials such as glass, nitrides, oxides, and
other metals the adhesion is poor. In addition, the thermal expansion coefficient
mismatch between SU-8 and silicon or glass is large.
SU-8 has been applied to MEMS fabrication 52,53 for plastic molds or electro-
plated metal micromolds. Also SU-8 MEMS structures have been used for micro-
fluidic channels, and biological applications. 54

3.8 CONCLUSIONS

Three categories of micromachining fabrication technologies have been presented;
bulk micromachining, LIGA, and sacrificial surface micromachining.
Bulk micromachining is primarily a silicon-based technology that employs wet
chemical etches and reactive ion etches to fabricate devices with high aspect ratio.
Control of the bulk micromachining etches with techniques such as etch stops and

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