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2-22 MEMS: Design and Fabrication

2.10 III–V Materials

Galium arsenide (GaAs), indium phosphide (InP), and other III–V compounds are attractive electronic
materials for various types of sensors and optoelectronic devices. In general, III–V compounds have
favorable piezoelectric and optoelectric properties, high piezoresistive constants, and wide electronic
bandgaps (relative to Si). In addition, III–V materials can be deposited as ternary and quaternary alloys
that have lattice constants closely matched to the binary compounds from which they are derived (e.g.,
Al Ga As and GaAs), thus permitting the fabrication of a wide variety of heterostructures that facilitate
x 1–x
device performance. Although the III–V class of materials is quite large, this section of the chapter will
focus on GaAs and InP for MEMS applications.
Crystalline GaAs has a zinc-blend crystal structure. It has an electronic bandgap of 1.4 eV, enabling
GaAs electronic devices to function at temperatures as high as 350°C [Hjort et al., 1994]. High-quality
single-crystal wafers are commercially available, as are well developed metallorganic chemical vapor dep-
osition (MOCVD) and molecular beam epitaxy (MBE) growth processes for epitaxial layers of GaAs and
its alloys. GaAs does not outperform Si in terms of mechanical properties; however, its stiffness and frac-
ture toughness are still suitable for micromechanical devices. A favorable combination of mechanical and
electrical properties makes GaAs attractive for certain MEMS applications.
Micromachining of GaAs is relatively straightforward, as many of its lattice-matched ternary and qua-
ternary alloys have sufficiently varying chemical properties to allow their use as sacrificial layers. For
example, the most common ternary alloy for GaAs is Al Ga 1–x As. For values of x 0.5, etchants con-
x
taining mixtures of HF and H O etch Al Ga As without attacking GaAs. In contrast, etchants consist-
2 x 1–x
ing of NH OH and H O mixtures attack GaAs isotropically but do not etch Al Ga 1–x As, thereby enabling
2
2
x
4
the bulk micromachining of GaAs wafers with lattice-matched etch stops. An extensive review of III–V
etch processes can be found in Hjort (1996). By taking advantage of the single-crystal heterostructures
that can be formed on GaAs substrates, both surface micromachined and bulk micromachined devices
can be fabricated from GaAs. The list of devices is widely varying and includes comb-drive lateral
resonant structures [Hjort, 1996], pressure sensors [Fobelets et al., 1994; Dehe et al., 1995b], thermopile
sensors [Dehe et al., 1995a], and Fabry–Perot detectors [Dehe et al., 1998].
Micromachining of InP closely resembles the techniques used for GaAs. Many of the properties of InP
are similar to GaAs in terms of crystal structure, mechanical stiffness, and hardness; however, the optical
properties of InP make it particularly attractive for micro-optomechanical devices to be used in the 1.3
to 1.55 µm wavelengths [Seassal et al., 1996]. Like GaAs, single-crystal wafers of InP are readily available.
Ternary and quaternary lattice-matched alloys of InP include InGaAs, InAlAs, InGaAsP, and InGaAlAs
compounds; like GaAs, some of these can be used as either etch stop and/or sacrificial layers, depending
on the etch chemistry. For instance, InP structural layers deposited on In0.53Al0.47As sacrificial layers
can be released using C H O :H O :H O etchants. At the same time, InP films and substrates can be
6 8 7 2 2 2
etched in HCl:H O-based solutions with In0.53Ga0.47As films as etch stops. A comprehensive list of wet
2
chemical etches for InP and related alloys is reviewed in Hjort (1996). InP-based micromachining tech-
niques have been used to fabricate multi-air-gap filters [Leclerq et al., 1998], bridge structures [Seassal
et al., 1996], and torsional membranes [Dehe et al., 1998] from InP and its related alloys.

2.11 Piezoelectric Materials

Piezoelectric materials play an important role in MEMS technology, mainly for mechanical actuation but
also to a lesser extent for sensing applications. In a piezoelectric material, mechanical stress polarizes the
material, which results in the production of an electric field. The effect also works in reverse; that is, an
applied electric field acts to produce a mechanical strain. Many materials retain some sort of piezoelec-
tric behavior, such as quartz, GaAs, and ZnO, to name a few. Recent work in MEMS has focused on the
development of the compound lead zirconate titanate, Pb(Zr Ti )O (PZT). PZT is attractive because it
1–x
3
x
has high piezoelectric constants that lead to high mechanical transduction.

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