Page 42 - An Introduction to Microelectromechanical Systems Engineering

Other Materials and Substrates 21

intermediate nickel or platinum layer are normally used to solder with silver-tin or
tin-lead alloys. For applications requiring transparent electrodes, such as liquid-
crystal displays, indium-tin-oxide (ITO) meets the requirements. Finally, Permal-
loy™ has been explored as a material for thin magnetic cores.

Polymers
Polymers, in the form of polyimides or photoresist, can be deposited with varying
thicknesses from a few nanometers to hundreds of microns. Standard photoresist is
spin-coated to a thickness of 1 µm to10 µm, but special photoresists such as the
epoxy-based SU-8 [6] can form layers up to 100 µm thick. Hardening of the resist
under ultraviolet light produces rigid structures. Spin-on organic polymers are
generally limited in their application as a permanent part of MEMS devices because
they shrink substantially as the solvent evaporates, and because they cannot sustain
temperatures above 200°C. Because of their unique absorption and adsorption
properties, polymers have gained acceptance in the sensing of chemical gases and
humidity [7].

Other Materials and Substrates

Over the years, micromachining methods have been applied to a variety of sub-
strates to fabricate passive microstructures as well as transducers. Fabrication
processes for glass and quartz are mature and well established, but for other materi-
als, such as silicon carbide, new techniques are being explored and developed. In the
process, these activities add breadth to micromachining technology and enrich the
inventory of available tools. The following sections briefly review the use of a few
materials other than silicon.

Glass and Fused Quartz Substrates
Glass is without a doubt a companion material to silicon; the two are bonded
together figuratively and literally in many ways. Silicon originates from processed
and purified silicates (a form of glass), and silicon can be made to bond electrostati-
®
cally to Pyrex glass substrates—a process called anodic bonding and common in
the making of pressure sensors. But like all relatives, differences remain. Glasses
generally have different coefficients of thermal expansion than silicon (fused quartz
is lower, while window glass is higher), resulting in interfacial stresses between
bonded silicon and glass substrates.
Micromachining of glass and fused quartz (amorphous silicon dioxide) sub-
strates is practical in special applications, such as when an optically transparent or
an electrically insulating substrate is required. Crystalline quartz (as opposed to
fused quartz) also has the distinct property of being piezoelectric and is used for
some MEMS devices. However, micromachining of glass or quartz is limited in
scope relative to silicon. Etching in HF or ultrasonic drilling typically yields coarsely
defined features with poor edge control. Thin metal films can be readily deposited
on glass or quartz substrates and defined using standard lithographic techniques.
Channels microfabricated in glass substrates with thin metal microelectrodes have
been useful in making capillaries for miniaturized biochemical analysis systems.