Page 168 - Microsensors, MEMS and Smart Devices - Gardner Varadhan and Awadelkarim

148 SILICON MICROMACHINING: SURFACE
m mrm

Si substrate Si substrate

(a) (b) Poly-Si

Silicon dioxide
Si substrate Si substrate

(c) (d)
Figure 6.3 Process flow for a polysilicon cantilever with a dimple anchored directly to a silicon
substrate
anchor hole, it is not possible to pattern both openings simultaneously using the same
mask. The preferable mask sequence for the patterning of the two openings is first the
patterning of the bushing mould followed by the anchor region definition since the latter
opening is the deepest of the openings. The rest of the process proceeds as described in
the one-mask and two-mask processes.
The process in which an insulating layer is incorporated between the cantilever and
substrate is illustrated in Worked Example 6.1. The insulating layer that works very well
with the poly-Si-oxide combination is silicon nitride (see also Section 6.3). This process
and three other worked examples are now presented in turn:

• Freestanding polysilicon beam
• Linear motion microactuator
• Rotor on a centre-pin bearing
• Rotor on a flange bearing

Worked Example E6.1A: Freestanding Poly-Si Beam

Objective (A):
The objective is to fabricate a poly-Si freestanding beam that rests on the surface of a
silicon wafer but is raised above it by a silicon nitride-insulating step.

Process Flow (A):
A layer of silicon nitride is first deposited by LPCVD on the surface of a silicon wafer
(Figure 6.4(a)). The thickness of the nitride film corresponds to the height of the step
on which the freestanding beam base is to rest. This nitride film also acts as a protective
layer for the silicon substrate. A layer of sacrificial SiO 2 is then deposited by chemical
vapour deposition (CVD) on top of the nitride layer (Figure 6.4(b)) and patterned as
shown in Figure 6.4(c). This patterned oxide island is of a thickness that is equal to the
height above the nitride-layer surface of the freestanding beam. Poly-Si is then deposited
by LPCVD on the patterned oxide as shown in Figure 6.4(d). When the sacrificial SiO 2
island is laterally etched, the freestanding beam shown in Figure 6.4(e) is finally created.