17
2
Substrate for MEMS
2.1 Introduction
Microelectromechanical system (MEMS) is a triumph of twenty-rst
centuries and revolutionized the semiconductor industry by combining
the microelectronics with micromachining technology. Though the term
‘mechanical’ is associated with MEMS, but in true sense, micromachined
devices should not contain mechanical part always. But the purpose of
MEMS technology is miniaturization that is needed to be addressed.
Basically, MEMSis a platform on top of which some common microscopic
mecha nical parts, e.g. channels, holes, cantilevers, membranes, cavities and
other structures, can be fabricated. Though micromachining is associated
with MEMS, still the structures are not machined. Rather they are created
using microfabrication technology suitable for batch processing for the
integrated circuits.
The potentiality of MEMS fabrication is batch processing. Batch processing
is highly economical; therefore, MEMS adds to economy in microelectronics
industry.
The emphasis is given while choosing the MEMS for their mechanical
properties than electrical. Although it depends on the particular applica-
tion, mechanical properties have been encountered in MEMS such as high
stiffness, high fracture strength, fracture toughness and high-temperature
stability or chemical inertness.
The basic building block of MEMS devices is the substrate which is nothing
but an object with macroscopic surface nish. In semiconductor electronics,
the substrate is nothing but a slice of single crystal silicon which is com-
monly known as wafer. Wafer can also be made of other crystalline materials,
e.g. quartz, alumina, GaAs and so on. These wafers should have a material
quality, as high as possible, and at the same time, they should be cheaper to
manufacture.
Among them the main advantage of using semiconductor (Si, Ge, GaAs)
as a substrate material is that it can be used as a semiconductor as well as an
insulator depending on the application in microelectronic industry. This ex-
ibility has been achieved by the technique termed doping in which foreign
18 MEMS and Nanotechnology for Gas Sensors
materials are added to the semiconductor to convert them from semiconduc-
tor to the electrical conductor. The physics behind it is already elaborated in
different books, so this is not included in this chapter.
From the technical point of view, the use of semiconductor as substrate
material confers the integration of MEMS and CMOS structures into a truly
monolithic device which is highly challenging; as all the processes such as
etching, diffusion, deposition, etc., have already been established. However,
much evolution continues to be made.
2.2 Silicon: The Base
2.2.1 Silicon as a Semiconductor
Single-crystal silicon is the principal material in the semiconductor industry.
Other mostly used silicon compounds are SiO
2
, SiC, Si
3
N
4
and polysilicon.
The polymers can also be used as substrate materials in MEMS and micro-
systems technology.
Among all, silicon is an ideal substrate material for MEMS. Silicon (Si) is
the most abundant material on earth. Most of the time, it is found in com-
pounds with other elements as a mixture.
Silicon is a popular semiconductor material and has already replaced other
substrate materials due to the following reasons:
1. It is mechanically stable and is used in already advanced microfab-
rication technology.
2. It is lighter than Al and harder than steel. It has about the same
Young’s modulus as steel (~2 × 10
5
MPa), but is as light as aluminium
with a density of about 2.3 g/cm
3
.
3. Silicon is almost an ideal structural material. Miniaturized mechani-
cal devices can be realized on silicon with high precision.
4. It has a melting point at 1400°C, which is about twice than that of
aluminium. The high melting point makes silicon dimensionally
stable even at high temperature.
5. Its thermal expansion coefcient is about 8 times smaller than
that of steel and is more than 10 times smaller than that of
aluminium.
6. Virtually, silicon has no mechanical hysteresis. It is thus an ideal
candidate material for sensors and actuators.
7. Silicon wafers are supernished, thus not suitable for coating. The
extra thin-lm layers should be integral structural parts of silicon,
performing exact electromechanical functions.

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