Biomedical Engineering Reference
In-Depth Information
produced from the same monomer, modified only by the substitution of a chlorine atom for one of the
aromatic hydrogens. Parylene C has a useful combination of electrical and physical properties, as well
as a very low permeability to moisture and other corrosive gases. Parylene C is also able to provide
a conformal insulation. Parylene D is modified from the same monomer by the substitution of the
chlorine atom for two of the aromatic hydrogens. Parylene D is similar in properties to parylene C with
the added ability to withstand higher temperatures. Deposition rates are fast, especially for parylene C,
which is normally deposited at a rate of about 10
m/min. The deposition rates of both parylene N and
parylene D are slower. Parylene can be used in microfluidic devices as a structural material, which
offers low Young's modulus. Such a soft material is needed in microvalves and micropumps.
Furthermore, parylene coating can improve the biocompatibility of a microfluidic device.
Polymeric surface micromachining perfectly suits for the fabrication of closed microchannels.
Both the structural layer and the sacrificial layer can be made of polymers. The typical fabrication
process is shown in Fig. 4.19 (a). To start with, the sacrificial polymer is spin-coated on the substrate,
which can be silicon or glass. The channel height is determined by the thickness of this layer, which in
turn is controlled by the viscosity of the solution and the spin speed. Since photoresist will be etched in
oxygen plasma, a metal layer is sputtered over the sacrificial layer as a mask ( Fig. 4.19 (a1)). The metal
mask allows conventional photolithography, where channel patterns are transferred to the mask
( Fig. 4.19 (a2)). The sacrificial layer is then structured by RIE with oxygen plasma ( Fig. 4.19 (a3)).
After removing the metallic mask, the structural polymer is deposited over the sacrificial structures
( Fig. 4.19 (a4)). In the final step, the sacrificial polymer decomposes into volatile products at elevated
temperatures, and leaves behind the microchannel [77] .
In the above-mentioned process, the sacrificial polymer should easily decompose at a temperature
lower than the glass temperature of wall materials. For instance, polynorbornene (PNB) is a good
sacrificial polymer [78] . The decomposition temperatures of PNB are between 370 C and 425 C. In
this case, silicon dioxide and silicon nitride are ideal encapsulation materials at these relatively high
temperatures. If polymeric channel walls are needed, polyimides, such as Amoco Ultradel 7501,
Dupont PI-2611, and Dupont PI-2734, are ideal for this purpose, because of their high glass transition
temperature of over 400 C [78] .
Polycarbonates, such as polyethylene carbonate (PEC) and polypropylene carbonate (PPC),
offer relatively low decomposition temperature on the order of 200
m
300 C [77] .Thelow
decomposition temperature is needed for structural materials with less thermal stability. Inorganic
glass, silicon dioxide, thermoplastic polymers, and thermoset polymers can be used as structural
materials [77] .
Micromixers used in life sciences may need a biocompatible coating for their inner walls.
Fig. 3.19(b) shows the fabrication processes of such microchannels [85] . For instance, the biocom-
patible material can be parylene C. First, parylene is vapor-deposited on a silicon substrate, which is
covered by a nitride/oxide barrier layer ( Fig. 4.19 (b,1)). Thick-film resist AZ4620 is used as the
sacrificial material. After photolithography, developing, and hard bake of the resist structures ( Figs.
4.19 (b,2)), a second parylene layer is deposited (Figs. 3.19(b,3)). After roughening the parylene
surface with oxygen plasma, photosensitive polyimide is spin-coated as a structural layer on top of the
second parylene layer. Next, polyimide is exposed and developed. In order to open the fluidic access
from the front side, the top parylene layer is etched in oxygen plasma with an aluminum mask
( Fig. 4.19 (b,4)). In the last step, the sacrificial layer is removed with acetone. The resulting micro-
channels are optically transparent and hermetic ( Fig. 4.19 (b,5)).
e
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