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2-16 MEMS: Design and Fabrication

2.7 Metals

Metals are used in many different capacities ranging from hard etch masks and thin film conducting
interconnects to structural elements in microsensors and microactuators. Metallic thin films can be
deposited using a wide range of deposition techniques, the most common being evaporation, sputtering,
CVD, and electroplating. Such a wide range of deposition methods makes metal thin films one of the
most versatile classes of materials used in MEMS devices. A complete review would constitute a chapter
in itself; the following illustrative examples are included to give the reader an idea of how different metal
thin films can be used.
Aluminum (Al) is probably the most widely used metal in microfabricated devices. In MEMS, Al thin
films can be used in conjunction with polymers such as polyimide because the films can be sputter-
deposited at low temperatures. In most cases, Al is used as a structural layer; however, Al can be used as
a sacrificial layer as well. The polyimide/aluminum combination as structural and sacrificial materials,
respectively, has also been demonstrated to be effective for surface micromachining [Schmidt et al., 1988;
Mahadevan et al., 1990]. In this case, acid-based Al etchants can be used to dissolve the Al sacrificial layer.
A unique feature of this material system is that polyimide is significantly more compliant than polysili-
con and silicon nitride (e.g., its elastic modulus is nearly 50 times smaller). At the same time, polyimide
can withstand large strains (up to 100% for some chemistries) before fracture. Finally, because both
polyimide and Al can be processed at low temperatures (e.g., below 400°C), this material system can be
used subsequent to the fabrication of ICs on the wafer. A drawback of polyimide is its viscoelasticity
(i.e., it creeps).
Tungsten (deposited by CVD) as a structural material and silicon dioxide as a sacrificial material have
also been used for surface micromachining [Chen and MacDonald, 1991]. In this case, HF is used for
removing the sacrificial oxide. In conjunction with high-aspect-ratio processes, nickel and copper are
being used as structural layers with polyimide and other metals (e.g., chromium) as the sacrificial layers.
The study of many of these material systems has been either limited or is in the preliminary stages; as a
result, their benefits are yet to be determined.
Metal thin films are among the most versatile MEMS materials, as alloys of certain metallic elements
exhibit a behavior known as the shape-memory effect. The shape-memory effect relies on the reversible
transformation from a ductile martensite phase to a stiff austenite phase upon the application of heat.
The reversible nature of this phase change allows the shape-memory effect to be used as an actuation
mechanism. Moreover, it has been found that high forces and strains can be generated from shape-memory
thin films at reasonable power inputs, thus enabling shape memory actuation to be used in MEMS-based
microfluidic devices such as microvalves and micropumps. Alloys of Ti and Ni, collectively known as
TiNi, are among the most popular shape-memory alloys owing to their high actuation work densities
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(reported to be up to 50MJ/m ) and large bandwidth (up to 0.1kHz) [Shih et al., 2001]. TiNi is also
attractive because conventional sputtering techniques can be employed to deposit thin films of the alloy,
as detailed in a recent report [Shih et al., 2001]. In this study, TiNi films deposited by two methods —
cosputtering elemental Ti and Ni targets and cosputtering TiNi alloy and elemental Ti targets — were
compared for use in microfabricated shape-memory actuators. In each case, the objective was to estab-
lish conditions so that films with the proper stoichiometry, and hence phase transition temperature,
could be maintained. The sputtering tool was equipped with a substrate heater in order to deposit films
on heated substrates as well as to anneal the films in vacuum after deposition. It was reported that cosput-
tering from TiNi and Ti targets produced better films than cosputtering from Ni and Ti targets, due
to process variations related to roughening of the Ni target. The TiNi/Ti cosputtering process has been
successfully used as an actuation material in a silicon spring-based microvalve [Hahm et al., 2000].
Use of thin-film metal alloys in magnetic actuator systems is yet another example of the versatility of
metallic materials in MEMS. From a physical perspective, magnetic actuation is fundamentally the same
in the microscopic and macroscopic domains, with the main difference being that process constraints
limit the design options of microscale devices. Magnetic actuation in microdevices generally requires the
magnetic layers to be relatively thick (tens to hundreds of microns), so as to create structures that can be

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