31
3
Deposition
Deposition is one of the main fabrication tools used in microelectromechani-
cal system (MEMS) fabrication to deposit thin lms of materials. Deposition
actually changes the surface properties of the base material on which it is
deposited. In this chapter, we discuss the deposition of thin-lm thickness
between a few nanometres and about 100 µm. In MEMS technology, the lm
is patterned and can be subsequently etched away using the steps elaborated
in the lithography chapter of this book.
MEMS deposition technology can be categorized into two ways. The rst
one is responsible for chemical reaction, i.e. chemical vapour deposition
(CVD), electrodeposition, epitaxial growth, etc.
The second deposition process is due to physical reaction, i.e. physical
vapour deposition (PVD). It is not possible to cover the whole thing in a sin-
gle chapter; still, the major parts are covered to give the reader an idea of the
overall concept.
3.1 Physical Vapour Deposition
PVD is basically applied to deposit thin lm of different materials onto a
substrate. The substrate may be a glass or alumina or silicon. The most com-
mon methods of PVD of metals are thermal evaporation, e-beam evapora-
tion, plasma deposition and sputtering. The structure of the thin lm can
be tailored by varying the deposition technique. Metals and different metal
compounds can be deposited by PVD.
Evaporation is one of the easiest routes of PVD. In this process, the source
material is heated above its melting point in a vacuum chamber. The evap-
orated atoms pass with a high velocity from source to target and follow
straight-line trajectories. The sources can be melted by various methods. One
is by resistive heating or radio frequency (RF) heating, or through a focused
electron beam. Among all, thermal and e-beam evaporations are extensively
used till date due to their simplicity in nature. But presently, sputtering tech-
nique is used in modern ULSI (ultra-large scale integration: more than one
million components per chip) circuit.
Atomic layer deposition (ALD) is another important PVD technique and
is mainly used for high-aspect-ratio structures. The deposition of materi-
als is reduced to one monolayer at a time. After successive deposition, an
32 MEMS and Nanotechnology for Gas Sensors
extremely conformal, defect-free layer is formed on the high-aspect-ratio
structures. In this technique, precursor reacts with the surface forming only
a monolayer of the material. The precursor is injected into the chamber, and
then the surface is again hydroxylated with water vapour or oxygen fol-
lowed by another purge. These two steps are then repeated until the desired
thickness of the material is achieved. ALD has a vast array of applications
from semiconductors, MEMS, nanostructures and optics to wear-resistant
coatings.
A brief study of the different thin-lm deposition techniques is explained
in the following.
3.1.1 Vacuum Technology for MEMS
Deposition and vacuum, these two terms, are closely related because most
of the deposition process occurs in vacuum condition. It is the only way to
achieve the required quality. Particularly, for thin-lm deposition, the study
of vacuum technology is extremely important and has been used since the
last two decades. Several authors have worked out on vacuum technology
[1], and being a very specic topic of research, the detailed discussion is not
included here; this section will be restricted to a discussion of applications in
the important elds of coating technology.
In industrial vacuum processes, four pressure regions can be
distinguished:
Rough vacuum 10
5
–10
2
Pa
Medium vacuum 10
2
–10
−1
Pa
High vacuum 10
−1
–10
−5
Pa
Ultra-high vacuum 10
−5
–10
−8
Pa
Right now, the elds of vacuum technology are manifold:
Electronics (e.g. cathode ray tube [CRT], drying of electric
components)
Deposition techniques (e.g. evaporation, sputtering)
Food and pharmaceutical industry
Chemical analysis techniques (e.g. Rutherford backscattering spec-
trometry [RBS], scanning electron microscope [SEM], secondary ion
mass spectrometry [SIMS], atomic emission spectroscopy [AES])
There are many advantages of using vacuum. A very good quality of sur-
face morphology of the deposited lm has been observed while using vac-
uum processes. The properties of deposited thin lms can be measured by
insitu monitoring technique. There is a high rate of repeatability, which is

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