Fabrication technologies - Micromixers Fundamentals Design and Fabrication - page 148

(a) (b)

FIGURE 4.18
Examples of devices made by polymeric surface micromachining: (a) microvalve, (b) microgripper.

Developing both layers results in a three-dimensional valve structure with spring beams, a valve disk,
and a sealing ring. Underetching silicon with KOH releases the valve. Circular access holes were
placed on the structure for faster release. The smooth contours of the design help to arrest surface stress
and avoid cracks in the structure.
For many applications, a metal layer on the structural polymeric material is needed. The metal layer
can be structured to form electrodes and heaters. Figure 4.18 (b) shows an SU-8 microgripper, which has
a thin metal layer on top acting as a heater [72] . Instead of silicon, a thin layer of polystyrene can be
used as sacrificial layer. The polymeric sacrificial material can be dissolved by organic solvents such as
toluene. In contrast to KOH for sacrificial silicon etching, solvent does not attack the thin metal layer.

4.2.3.2 Polyimide
Polyimide is available as photoresists such as Proimide 348 or 349 (Ciba Geigy) or PI-2732
(DuPont). Coating a single polyimide layer can result in a film thickness up to 40

m

m. This relatively
thick film allows the use of photosensitive polyimide for the same purpose as other thick resists, such
as SU-8 [73] .
Fluorinated polyimide is optically transparent. This material can be machined by RIE. During an
RIE process, fluorine radicals are released from the fluorinated polyimide and act as etchants [74] .
Polyimide can work as a substrate material. Metals such as aluminum, titanium, and platinum can
be sputtered on it [75] . Similar to other polymers, polyimide can be etched with RIE in oxygen plasma.
Combining photolithography, RIE, and lamination, complex channel structures with metal electrodes
can be fabricated in polyimide [76] .

4.2.3.3 Parylene
Parylene is a polymer that can be deposited with CVD at room temperature. The CVD process allows
coating a conformal film with a thickness ranging from several microns to several millimeters.
Parylene is available as Parylene N, Parylene C, and Parylene D.
Parylene N is poly-paraxylylene, which is a good dielectric. Parylene N has a very low dissipation
factor, high dielectric strength, and a frequency-independent dielectric constant. Parylene C is