Medical Applications of Micro-Electro-Mechanical Systems (MEMS) Technology - Biomaterials Science: An Integrated Clinical and Engineering Approach - page 37

with tighter dimensional control than is possible with isotropic etching. It
also provides for two-sided processing to embody self-isolated structures
where only one side is exposed to the environment. This assists in the pack-
aging of the device and is very useful for MEMS devices exposed to harsh
environments, such as pressure sensors.
Anisotropic etching techniques have been around for over 25 years and
are commonly used in the manufacturing of silicon pressure sensors as well
as bulk micromachined accelerometers.
Figure 3.2 is an illustration of some of the shapes that are possible using
anisotropic wet etching of a <100> oriented silicon substrate, including an
inverted pyramidal and a flat bottomed trapezoidal etch pit. Note that the
shape of the etch pattern is determined primarily by the slower etching <111>
planes. Figure 3.3a and Figure 3.3b are SEM photographs of a silicon substrate
after an anisotropic wet etching. Figure 3.3a shows a trapezoidal etch pit that
has been subsequently diced across the etch pit, and Figure 3.3b shows the
backside of a thin membrane that could be used to make a pressure sensor.
It is important to note that the etch profiles shown in these figures are only
for a <100> oriented silicon wafer; substrates with other crystallographic ori-
entations will exhibit different shapes. Occasionally, substrates with other
orientations are used in MEMS fabrication, but given the cost, lead times,
and availability, the vast majority of substrates used in bulk micromachining
have <100> orientation.

Boron-doped
Si membrane

{100}

Frontside Mask

{111}

54.74°

{111}
Backside Mask

<100>

Self-limiting etches

{111}

FIGuRE 3.2
The shape of the etch profiles of a <100> oriented silicon substrate after immersion in an aniso-
tropic wet etchant solution.