Fabrication technologies - Micromixers Fundamentals Design and Fabrication - page 155

assistance of a solvent, which wets the bonding surfaces. Bonding is accomplished after the solvent
evaporates.
Ceramic green tapes and metal sheets structured by serial techniques can be directly bonded
together at high pressure and high temperatures. Ceramic green tapes are typically bonded at 138
bars, 70 C for 10 min [105] . Stainless-steel sheets are typically bonded at 276 bars, 920 Cfor
4h [106] .

4.4.3 Adhesive bonding
Adhesive bonding uses an intermediate layer to “glue” the substrate. Depending on substrate materials
and applications, the intermediate layer can be glass, epoxies, photoresists, or other polymers. A thin
intermediate glass layer can thermally bond silicon wafers. Glass frits with relatively low sealing
temperatures ranging from 400 C to 650 C are commercially available. The glass layer can be
sprayed, screen-printed, or sputtered on the substrate. Annealing the stack at sealing temperatures
makes the glass layer melt and flow. Cooling down to room temperature results in a strong bond
between two substrates [107] . A number of epoxies [108] , UV-curable epoxies [109] , and photoresists
can be used for adhesive bonding. SU-8 is used in many microfluidic applications as both spacer and
adhesive layers. The advantage of using polymers as an intermediate layer is the low process
temperature. These low packaging temperatures are needed for many devices, which have metals and
alloys with low melting temperatures. The other advantage is that adhesive bonding is not limited to
silicon and can be used for all types of substrate material.

4.4.4 Eutectic bonding
Eutectic bonding is a common packaging technique in electronics. Gold silicon eutectic bonding is

achieved at a relatively low temperature of 363 C. A thin gold film can be sputtered on the silicon
surface for this purpose. Furthermore, a gold

e

silicon preform with composition close to the eutectic
point can also be used as the intermediate layer.

e

4.5 CONCLUSIONS
This chapter gives a short review on available micromachining technologies for silicon-based,
metallic, and polymeric micromixers. Because a complete review on the technology for each type
of material could cover hundreds of references, this chapter only summarizes the most important
points on the topics, and in many cases cites the topical review on each type of material. In general,
silicon-based technologies are the most established techniques with commercially available
equipments. Applications in rough environments and operation conditions require a tougher
material than silicon. For such applications, diamond-based and silicon carbide-based devices are
the better candidates. Applications in analytical chemistry and biomedical fields would require
a large device area, which is not economical for silicon. Furthermore, silicon and silicon-based
materials are not compatible for many chemical and biochemical applications. Polymeric devices
are the real alternatives for silicon-based counterparts. Besides simple devices with only micro-
channel networks, polymeric devices with freely movable components are also possible. Metallic
devices are other alternatives in microscale. The combination of all the available technologies from