Page 44 - An Introduction to Microelectromechanical Systems Engineering

Other Materials and Substrates 23

III-V compound semiconductors is a practical way to integrate RF switches, anten-
nas, and other custom high-frequency components with ultra-high-speed electronic
devices for wireless telecommunications.

Polymers

Polymers are long chains of carbon (or sometimes silicon) atoms with various
chemical side groups attached to the carbon [13]. If the chains are not crosslinked
by covalent bonds, they are able to move relative to each other at elevated tempera-
ture under applied stress. Such materials reharden upon cooling and are called
thermoplastics. The temperature above which flow readily occurs is the glass
transition temperature, which varies with the length of the molecules and the type of
side groups.
PMMA [poly(methylmethacrylate)], polypropylene, polyvinyl chloride, acrylic,
and other thermoplastics are used in sheet form as a substrate for micromachining.
Heating above the glass transition temperature enables molding or embossing under
pressure from a master for some of these materials (described in Chapter 3). Layers
of polycarbonate and acrylic, with channels already formed in their surfaces by hot
embossing or conventional machining, have been thermally bonded together for
microfluidic systems. In MEMS, thick layers of PMMA have also been spin-coated
and used as a photoresist.
Polymer substrates have not been used as much as silicon in micromachining,
but have some advantages, perhaps the most important being lower cost. The proc-
essing temperatures allowed are much lower than for silicon and many glasses, but
suitable fabrication processes have been designed, particularly for biological appli-
cations. Polymers are in general less stiff than inorganic materials (see Table 2.1).
Polyimide is a material that is most often used in the form of sheets 7 to 125 µm
thick, but can also be spin-coated in films a few micrometers thick. It is sold by
DuPont High Performance Films of Circleville, Ohio, under the trade name Kap-
ton . Polyimide is relatively inert, is a good electrical insulator, and can be exposed
®
to a wide range of temperatures, roughly –250º to +400ºC, for at least a short time
[14]. In the electronics industry, polyimide has been used as a flexible substrate for
printed circuit boards and for hard disk drives. In micromachining, sheets have been
laser cut to form microfluidic devices, while spin-on films have been used as resists,
sacrificial layers, and a wafer-bonding adhesive.
Other polymers finding application in MEMS include parylenes and silicones.
Parylenes are deposited by chemical-vapor deposition to form a conformal coating.
There are several forms of parylene due to variations in the chemical structure [15].
Like polyimide, parylenes are fairly inert chemically and form a barrier to the flow
of water and other vapors. Silicones are different from most other polymers in that
the backbone chain of atoms is silicon rather than carbon. Silicones are very compli-
ant and have been used as the deformable membrane in valves [15], as well as being
a common die-attach material in packaging (see Chapter 8).

Shape-Memory Alloys
The shape-memory effect is a unique property of a special class of alloys that return
to a predetermined shape when heated above a critical transition temperature. The