Page 43 - An Introduction to Microelectromechanical Systems Engineering

22 Materials for MEMS

Silicon Carbide and Diamond

Silicon carbide and diamond continue to captivate the imagination of many in the
micromachining community. Both materials offer significant advantages, in particu-
lar hardness, high stiffness (high Young’s modulus), resistance to harsh chemical
environments, mechanical stability at high temperature, wide bandgap, and very
high thermal conductivity (see Table 2.1). Some micromachining in silicon carbide
[8] and diamond has been demonstrated; however, much remains to be studied
about both materials and their potential use in MEMS. An important feature of both
silicon carbide and diamond is that they exhibit piezoresistive properties. High-
temperature pressure sensors in silicon carbide substrates have been developed with
stable operation up to about 500°C.
Silicon carbide (SiC) has a number of possible crystal structures, including cubic
and hexagonal. Hexagonal crystalline SiC substrates are commercially available,
but they are very expensive and are available only in diameters up to 76 mm [9].
Cubic crystalline silicon carbide can be obtained by epitaxial growth directly on
silicon (which has the same cubic structure), but the material has a high density of
voids and dislocations due to mismatch in lattice spacing. Thin polycrystalline SiC
films deposited by chemical vapor deposition can be used as the structural layer for
surface micromachining (discussed in Chapter 3), with a sacrificial layer of silicon or
silicon dioxide [8]. Because etching SiC is so difficult, alternative methods of
forming a pattern, such as selective deposition and using a mold, have been
studied. Silicon carbide films have also been used as a coating material for harsh
environments.
Diamond is an even lesser-explored material than silicon carbide. Thin syn-
thetic polycrystalline diamond or “diamond-like carbon” films made with thick-
nesses up to a few microns can be formed using chemical vapor deposition.
Diamond has an extremely high ratio of Young’s modulus to density, giving vibrat-
ing structures made of diamond higher resonant frequencies than similar structures
made of other materials. In addition to the properties listed earlier, diamond films
are also good field emitters and have received extensive study as a source of elec-
trons for such applications as displays. Etching diamond films is even more difficult
than for silicon carbide, so alternative patterning methods such as selective deposi-
tion are used [9].

Gallium Arsenide and Other Group III-V Compound Semiconductors

Rather than ponder the utility of gallium arsenide (GaAs) and other group III-V
compounds (e.g., InP, AlGaAs, GaN) as alternate substrate materials to silicon, it is
perhaps more appropriate to think of micromachining as a set of tools that can pro-
vide solutions to issues specific to devices that currently can only be built in these
materials, in particular lasers and optical devices. In that regard, micromachining
becomes an application-specific toolbox whose main characteristic is to address
ways to enable new functions or enhance existing ones.
Micromechanical structures such as springs and bridges have been formed in
GaAs by both reactive ion etching [10] and orientation-dependent etching [11] (dis-
cussed in Chapter 3). Micromachining has also been used to incorporate structures
such as mirrors on the surface of III-V semiconductors to create new devices, includ-
ing tunable lasers [12]. Moreover, micromachining using GaAs and other group