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Materials for Microelectromechanical Systems 2-17

used to generate magnetic fields of sufficient strength to generate the desired actuation. To this end, mag-
netic materials are often deposited by thick-film methods such as electroplating. The thicknesses of these
layers often exceeds what can feasibly be patterned by etching, so plating is often conducted in microfab-
ricated molds usually made from X-ray-sensitive materials such as polymethylmethacrylate (PMMA).
The PMMA mold thickness can exceed several hundred microns, so X-rays are used as the exposure
source. In some cases, a thin-film seed layer is deposited by sputtering or other conventional means before
the plating process begins. At the completion of the plating process, the mold is dissolved, freeing
the metallic component. This process, commonly known as LIGA, has been used to produce high-
aspect-ratio structures such as microgears from NiFe magnetic alloys [Leith and Schwartz, 1999]. LIGA
is not restricted to the creation of magnetic actuator structures and, in fact, has been used to make such
structures as Ni fuel atomizers [Rajan et al., 1999]. In this application, Ni was selected for its desirable
chemical, wear, and temperature properties, not its magnetic properties.

2.8 Silicon Carbide

Use of Si as a mechanical and electrical material has enabled the development of MEMS for a wide range
of applications. Of course, use of MEMS is restricted by the physical properties of the material, which in
the case of Si-based MEMS limits the devices to operating temperatures of about 200°C in low-wear and
benign chemical environments. Therefore, alternate materials are necessary to extend the usefulness of
MEMS to areas classified as harsh environments. In a broad sense, harsh environments include all condi-
tions where use of Si is prohibited by its electrical, mechanical, and chemical properties. These would
include high-temperature, high-radiation, high-wear, and highly acidic and basic chemical environments.
To be a direct replacement for Si in such applications, the material would have to be a chemically inert,
extremely hard, temperature-insensitive, micromachinable semiconductor. These requirements pose sig-
nificant fabrication challenges, as micromachining requires the use of chemical and mechanical processes
to remove unwanted material. In general, a class of wide bandgap semiconductors that includes silicon
carbide (SiC) and diamond embodies the electrical, mechanical, and chemical properties required for
many harsh environment applications, but until recently these materials found little usefulness in MEMS
because the necessary micromachining processes did not exist. The following two sections review the
development of SiC and diamond for MEMS applications.
SiC has long been recognized as a semiconductor with potential for use in high-temperature and high-
power electronics. SiC is polymorphic, meaning that it exists in multiple crystalline structures, each shar-
ing a common stoichiometry. SiC exists in three main polytypes: cubic, hexagonal, and rhombehedral.
The cubic polytype, called 3C-SiC, has an electronic bandgap of 2.3 eV, which is over twice that of Si.
Numerous hexagonal and rhombehedral polytypes have been identified, the two most common being the
4H-SiC and 6H-SiC hexagonal polytypes. The electronic bandgap of 4H- and 6H-SiC is even higher than
3C-SiC, being 2.9 and 3.2 eV respectively. SiC in general has a high thermal conductivity, ranging from
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3.2 to 4.9 W/cm-K, and a high breakdown field (30 10 V/cm). SiC films can be doped to create n- and
p-type material. The stiffness of SiC is quite large relative to Si; with measured Young’s modulus values in
the range of 300 to 700GPa, it is very attractive for micromachined resonators and filters, as the resonant
frequency increases with increasing modulus. SiC is not etched in any wet chemistries commonly used in
Si micromachining. SiC can be etched in strong bases like KOH, but only at temperatures in excess of
600°C. SiC does not melt but rather sublimes at temperatures in excess of 1800°C. Single-crystal 4H- and
6H-SiC wafers are commercially available, although they are smaller (3 in diameter) and much more
expensive than Si. With this list of properties, it is little wonder why SiC is being actively researched for
MEMS applications.
SiC thin films can be grown or deposited using a number of different techniques. For high-quality single-
crystal films, APCVD and LPCVD processes are most commonly employed. The high crystal quality is
achieved by homoepitaxial growth of 4H- and 6H-SiC films on substrates of like crystal type. These
processes usually employ dual precursors to supply Si and C, with the common sources being SiH and
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