Page 23 - MEMS Mechanical Sensors

12 Materials and Fabrication Techniques

developed specifically for MEMS. The three essential elements in conventional
silicon processing are deposition, lithography, and etching. These are illustrated in
Figure 2.4. The common deposition processes, which include growth processes, are
oxidation, chemical vapor deposition, epitaxy, physical vapor deposition, diffu-
sion, and ion implantation. The types of lithography used are either optical or elec-
tron beam, and etching is done using either a wet or dry chemical etch process.
Many of these conventional techniques have been modified for MEMS purposes,
for example, the use of thick photoresists, grayscale lithography, or deep reactive
ion etching. Other processes and techniques not used in conventional integrated cir-
cuit fabrication have been developed specifically for MEMS, and these include sur-
face micromachining, wafer bonding, thick-film screen printing, electroplating,
porous silicon, LIGA (the German acronym for Lithographie, Galvansformung,
Abformung), and focused ion beam etching and deposition. For a more general ref-
erence covering MEMS fabrication techniques, see the book by Kovaks [3].

2.3.1 Deposition
2.3.1.1 Thermal Growth
Silicon dioxide is grown on silicon wafers in wet or dry oxygen ambient. This is
done in a furnace at temperatures in the range from 750°C to 1,200°C. For oxides
grown at atmospheric pressure the thickness of the oxide can be as small as 1.5 nm
or as large as 2 µm. For each micron of silicon dioxide grown, 0.45 µm of silicon is
consumed and this generates an appreciable compressive stress at the interface.
Furthermore, there is a large difference between the thermal expansion coefficients
of silicon and silicon dioxide, which leaves the oxide in compression after cool-
ing from the growth temperature, adding to the intrinsic stress arising during
growth. Stress is, of course, an important issue for MEMS mechanical devices and

Deposition Deposit
layer

Spin on
resist

Exposure
Lithography to UV light
through
mask

Develop

Etch
Etching
Strip resist

Figure 2.4 Illustration of the deposition, lithography, and etch processes.