Fabrication technologies - Micromixers Fundamentals Design and Fabrication - page 137

Table 4.6 Properties of Common Thick-Film Resists

Resist PMMA SU-8 AZ4562

Exposure type X-ray (0.2 2 nm) UV (365, 405, 435 nm) UV (365, 405, 435 nm)

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Light source Synchrotron facility Mercury lamp Mercury lamp

Mask substrate Beryllium (100 m m) Quartz (1.5 3 mm) Glass (1.5 3 mm)

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Titanium (2 mm) Glass (1.5 3 mm) Quartz (1.5 3 mm)

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Mask absorber Gold (10 15 m m) Chromium (0.5 m m) Chromium (0.5 m m)

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Maximum height 1,000 m m 250 m m 100 m m

Aspect ratio ~500 20 25 ~10

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Young's modulus (GPa) 2 3 4 5 —

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Poisson's ratio — 0.22 —

Glass temperature ( C) 100 > 200 —

4.2.1.3 Other thick-film resists
Another commercially available thick-film resist is AZ4562 (Clariant, Charlotte, North Carolina),
which is a positive photoresist. This resist belongs to the Novolak resist system, which is in common
with most commercially available positive resists. Using multilayer spin coating, thick resist layers up
to 100

m can be achieved. This photoresist has no oxygen sensitivity, but a high resistance to plasma
etching, good adhesion properties, and high-resolution capability [66] . AZ4562 is typically used
either as a mold for subsequent metal electroplating [67,68] or as master templates for micromolding.
Table 4.6 gives a summary of the parameters of PMMA, SU-8, and AZ4562.
AZ9260 is the other Novolak photoresist from Clariant, which exhibits a better transparency than
AZ4562, and, therefore, promises a better aspect ratio. Aspect ratios up to 15 are achieved with a film
thickness of 100

m

m is expected from this photoresist.
Ma-P100 (Microresist Technology, Berlin, Germany) is the other photoresist that can give structure
heights up to 100

m m [68] . A theoretical thickness of 150 m

m m. This photoresist has aspect ratios on the order of 5, poorer than that of the AZ

family [69] .

4.2.2 Polymeric bulk micromachining
In contrast to many other microdevices, micromixers are large, due to their usually long micro-
channels. The sample volume required for detectability needs relatively large reservoirs. Therefore, the
cost of the substrate material plays an important role for large-scale production. For the same surface
area and optical transparency, a glass substrate may cost 10

100 times more than a polymer substrate.
Besides the cost advantage, polymers are available with a wide range of properties. Surface properties
of polymers can be tailored for specific applications. As low-cost materials, polymers can be used
directly as mechanical materials. Their electrical and chemical properties are interesting for physical,
chemical, and biochemical sensing [5] . Polymer membranes and matrices are widely used in
macroscale for the separation of DNA and proteins [6] .
Polymers are organic materials consisting of macromolecules, which may have more than 1000
monomeric units. The crosslinking process of the monomers is triggered chemically by an initiator
substance, or physically by photons, pressure, or temperature. In a polymerization reaction, monomer

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