Fabrication technologies - Micromixers Fundamentals Design and Fabrication - page 151

sacrificial material. Due to the relatively slow growth rate, evaporation and sputtering are suitable for
the deposition of thin metallic films up to 10- m thickness. Thicker metallic layer can be achieved by

electroplating. A metallic layer can be patterned by a lift-off process or by chemical etching.
Subsequently, etching away the sacrificial materials releases the freestanding metallic structure.
Actuators are the key components of active micromixers. Sputtering and metallic micromachining
allow the design and integration of functional smart materials, such as permanent magnet films
[88,89] , piezoelectric films [90,91] , and shape memory alloy films [92] .
Micromagnets consisting of metal alloys such as SmCo 5 ,Sm 2 (Co, Fe, Zr) 17 ,Nd 2 Fe 17 ,Nd(Fe,
Ti) 1 2N x , PtCo/Ag, Pt/Fe, CoNiMnP, FeCrCo, and MnAl can be sputtered or electroplated on the
substrate. Further, magnetic powders, such as ferrite (Fe 2 O 3 ), can be mixed with a polymer, such as
polyimide or PDMS, to form a magnetic polymeric matrix. This magnetic polymer matrix can be
structured by the common polymeric techniques discussed above or screen-printed on a substrate
[88] . Magnetic materials open up potential applications in magnetic micromixers and MHD
micromixers.
Piezoelectric thin films are not metallic but belong to the class of smart materials for actuation in
active micromixers [90] . Piezoelectric ceramics, such as AlN and ZnO, can be sputtered. While ZnO
needs to be deposited at room temperature for high resistivity, AlN with low conductivity can be
deposited at high temperatures between 100 C and 900 C. AlN is more compatible to silicon-based
technology because of the large resistivity and large band gap of 6 eV. Other popular piezoelectric
materials for MEMS devices are ferroelectric thin films, such as lead zirconate titanate (PZT).
Ferroelectric thin films have the advantage of large piezoelectric coefficients. However, for the
composition of the material the deposition process is relatively complex and challenging [90] . Readers
may refer to a recent review by Doerey and Whatmore for more details on fabrication issues of thick-
film PZT [91] .
Shape memory alloy (SMA) films are another attractive metallic material for actuators in active
micromixers. SMA materials, such as TiNi, can be sputtered and structured with conventional
microtechniques. The main advantages of SMA are high power density, large displacement and large
forces, and relatively low operation voltages. However, SMA actuators are thermal actuators that are
associated with problems such as low energy efficiency, low dynamics, and large hysteresis. The
hysteresis behavior leads to nonlinear system behavior, which makes designing SMA-based micro-
actuators difficult. A number of microfluidic devices, such as micropumps and microvalves, have been
realized based on SMA thin films [92] .

m

4.3.2 LIGA
LIGA process is a combination of X-ray or thick-resist lithography with electroplating. The metallic
part usually works as the mold for further replication in polymers. However, the same process can be
used for the fabrication of metallic micromixers. The process starts with applying a PMMA layer on
the substrate. This process can be achieved by different methods, such as multiple spin coating,
lamination of prefabricated sheets, casting, and plasma polymerization. The PMMA layer is structured
by X-ray lithography. The etched PMMA part is subsequently used as a mold for electroplating of the
metallic structure. The metal in use is typically nickel or nickel iron alloy.

The need of synchrotron X-ray source and the consequent high cost prevent the widespread use of
the LIGA process. The relatively high aspect ratio achievable with SU-8 allows a low-cost alternative

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