Fabrication technologies - Micromixers Fundamentals Design and Fabrication - page 130

with silicon nitride. In a similar process, the channel is underetched, resulting in a suspended nitride
channel [30] .
The above approach can be further developed using silicon substrate directly as sacrificial material
[28] . The mold is fabricated in a handle wafer with bulk micromachining. The channel wall is defined
by deposition of nitride/oxide double layer ( Fig. 4.8 (b)). With silicon dioxide on top, the silicon wafer
is bonded anodically to a glass wafer. Etching away the silicon handle wafer releases the nitride/oxide
channel on glass. If the channel wall is too thin for certain applications, the surface of the structure on
glass can be coated with a thick polymer layer [31] .
Besides the above techniques, microchannels with reasonable heights can be fabricated in metals
with the process described in [32] . The process starts with deposition of a metal seed layer on the
substrate ( Fig. 4.8 (c)). A subsequent electroplating process defines the bottom wall of the channel.
Next, a thick-film photoresist, such as AZ4620, is deposited and developed to form the sacrificial
structure for the channel. Gold is then sputtered on the resist structure as the second seed layer.
Electroplating on this seed layer forms the sidewall and top wall of the channel. Etching the gold layer
exposes the sacrificial photoresist. Removing photoresist with acetone creates a hollow metal channel.
A similar technique was used in [33] to fabricate more sophisticated microfluidic devices, such as
microvalves.
4.1.4 Other materials
4.1.4.1 Diamond thin films
Silicon-based devices have poor mechanical and tribological properties. Due to the prominent surface
effects, microdevices usually avoid large deflection and extensive sliding as well as rolling contacts.
Compared to silicon, carbon has superior properties. For instance, the coefficient of friction of single-
crystal diamond is on the order 0.01, which makes the wear life of a diamond-coated surface four
orders of magnitude higher than silicon [34] . Diamond film may be a good candidate for making
micromixers for extreme conditions. Diamond microstructures can be fabricated using thin film
deposition. Diamond thin films made with chemical vapor deposition methods have polycrystalline
characteristics and are categorized as microcrystalline diamond (MCD) and ultrananocrystalline
diamond (UNCD). The grain sizes of MCD and UNCD are on the order of several micrometers and
nanometers, respectively.
The easiest method is to coat a silicon-based component with a thin diamond film. This method
utilizes the well-established silicon technology but provides components with superb surface prop-
erties. Microcomponents can be fabricated based on UNCD by selective deposition and lithographic
patterning.
Selective deposition can be achieved by controlling the seeding layer before deposition. The
growth of diamond films requires a seeding layer, which is formed by exposing the substrate to
a suspension of fine diamond particles. The seeding layer can be patterned by:
Selected seeding with a photoresist mask,
Using diamond-loaded photoresist and subsequent photolithographic patterning, and
Selective etching of the seeding layer.
Diamond film can be doped with nitrogen to become electrically conductive. Combining with
a sacrificial layer, these technologies allow making diamond-based devices in the same way as