Page 177 - Microsensors, MEMS and Smart Devices - Gardner Varadhan and Awadelkarim

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1. Polyimide has a small elastic modulus, which is about 50 times smaller than that of
polycrystalline silicon.
2. Polyimide can take large strains before fracture.
3. Both polyimide and aluminum can be prepared at relatively low temperatures
(<400°C).

However, the main disadvantage of this material system lies with polyimide in that it
has unfavourable viscoelastic characteristics (i.e. it tends to creep), and so devices may
exhibit considerable parametric drift.

6.3.3 Silicon Nitride/Polycrystalline Silicon and
Tungsten/Silicon Dioxide

In the third material system of silicon nitride/poly-Si, silicon nitride is used as the struc-
tural material and poly-Si as the sacrificial material. For this material system, silicon
anisotropic etchants such as potassium hydroxide (KOH) and ethylenediarnine pyrocate-
9
chol (EDP) are used to dissolve the poly-Si .
In the fourth material system of tungsten/oxide, tungsten deposited by CVD is used as
the structural material with the oxide as the sacrificial material. Here again, HF solution
is used to remove the sacrificial oxide.
Finally, we give a worked example in which silicon nitride is employed as the structural
material as before but as a variant and, unusually, aluminum is used as the sacrificial layer
instead of poly-Si.

Worked Example E6.5: Silicon Condenser Microphone 10

Objective:
The objective is to fabricate a silicon condenser microphone using silicon nitride as the
structural layer and aluminum as the sacrificial layer. In this case, gold is used to make
the electrical tracks.
Process Flow:

1. First, the backside and polished front side of a silicon wafer are covered with 1.8 jim
and 0.45 |um thermal oxides, respectively. A square window is then etched on the
oxide on the backside. A sacrificial layer of aluminum about 1.0 urn thick is then
evaporated onto the polished front side of the wafer and patterned, followed by
plasma-enhanced chemical vapour deposition (PECVD) of 1.3 urn of silicon nitride
(Figure 6.11 (a)).
2. The diaphragm-to-be silicon nitride film is provided with a 30 nm titanium adhesion
layer and a 30-nm gold electrode. Holes and V-grooves are then etched anisotropically
from the backside using KOH, and the KOH etch stops at the SiO2 etch-stop layer
(Figure 6.1 l(b)).

9
Details of these etches are given in Section 2.4.
10
For details see Scheeper et al. (1992).