Page 81 - An Introduction to Microelectromechanical Systems Engineering

60 Processes for Micromachining

The process may be stopped at this point with a metal microstructure suitable
for some purposes. Alternatively, the metal can be used as a mold for plastic parts
(the “A” in LIGA).
Precision gears and other microstructures have been fabricated using LIGA, but
the method is considered expensive because of the requirement to use collimated
x-ray irradiation available only from synchrotrons. Mold formation using opti-
cal lithography is often called “poor man’s LIGA.” Guckel [23] provides addi-
tional details on the molding of high aspect ratio structures fabricated with x-ray
lithography.
In a variation known as electroforming, the plated metal is peeled off of the sub-
strate and is the useful structure. Examples of electroformed products are electric
shaver screens and some ink-jet heads.

Supercritical Drying
The final step of many micromachining processes is the removal of a sacrificial layer
(e.g., using hydrofluoric acid to etch 1 µm of silicon dioxide from under a polysilicon
beam). After rinsing, the water must be dried from the wafer. If a freestanding struc-
ture overhangs the substrate, surface tension forms a meniscus of water between the
two (see Figure 3.20). As the water dries, its volume (and hence thickness) decreases.
If the structure is compliant, as is usually the case in surface micromachining, it is
pulled down, contacting the substrate. If a sufficiently large, smooth area of the
structure makes contact, it can stick, which is known as stiction in the micromachin-
ing community. Such stuck structures can often be freed by pushing with a probe tip,
but this is hardly suitable for production.
A solution to avoid stiction after release is supercritical drying, also known as
critical-point drying [24]. In this process, the wafer is moved without drying into
methanol, which is miscible with the small amount of water left on the wafer during
transfer. The wafer is then placed in a pressure chamber, covered by methanol. Liq-
uid carbon dioxide, which is miscible with methanol, is flowed into the chamber at a
pressure of about 7.5–9 MPa as the methanol/carbon dioxide mixture is drained out
of the bottom. After a few minutes, only carbon dioxide is left in the chamber. The
chamber is then heated from room temperature (near 20°C) to about 35°C, which
also increases the pressure (see Figure 3.21). The carbon dioxide has now surpassed
the critical point [31.1°C, 7.39 MPa (1071 psia)] and is in the supercritical region, in
which liquid and gas are indistinguishable. Finally, the carbon dioxide is vented off.
As the pressure drops, the carbon dioxide in the chamber transitions from a super-
critical fluid to a gas with only one phase ever being present, thus preventing the

Freestanding cantilever

Water meniscus
(a) (b) (c)
Figure 3.20 Pull-down of a compliant freestanding structure (a cantilever) due to surface tension
during drying: (a) water completely fills the volume under the structure; (b) part of the water
volume has dried; and (c) most of the water volume has dried, with surface tension pulling the
structure down until it touches the substrate.