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MEMS Fabrication 45

The details and types of etch chemistries involved in plasma etching are varied and
quite complex. This topic is too voluminous to be discussed in detail here, but there
exist a number of excellent references on this subject. 15 The proper choice of these
chemistries produces various etch rates and selectivity of material etch rates, which is
essential to the integration of processes to produce microelectronics or MEMS devices.
Fluoride etch chemistries is one of the most widely studied for silicon etches. Equations
(3.3), (3.4), and (3.5) illustrate some of the fluoride reactions involved in the etching of
silicon, silicon dioxide, and silicon nitride, respectively. There are a number of feed
gases that can produce the free radicals involved in these reactions:

Si þ 4F ! SiF 4 (3:3)
þ
3SiO 2 þ 4CF ! 2CO þ 2CO 2 þ 3SiF 4 (3:4)
3

Si 3 N 4 þ 12F ! 3SiF 4 þ 2N 2 (3:5)
The anisotropy of the plasma etch can be increased by the formation of nonvolatile
fluorocarbons that deposit on the sidewalls as seen in Figure 3.7. This process is

Deposit and
pattern the
mask

Initial
deposition

Volatile etch product
Neutral
Ion

Initial
etch

Deposition

Neutral Volatile etch products
Ion
Nonvolatile sidewall deposits

Next
etch
cycle
FIGURE 3.7 Schematic of sidewall polymerization to enhance anisotropic etching.

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