Fabrication technologies - Micromixers Fundamentals Design and Fabrication - page 152

to standard LIGA. The LIGA process with SU-8 is often called UV-LIGA or “poor man's LIGA” [93] .
For more details on LIGA technology and its application, readers may refer to a recent review by
Malek and Saile [94] .
4.3.3 Micro-electro-discharge machining
Micro-electro-discharge machining (micro-EDM) uses erosive effects of electrostatic discharge
between an electrode and an electrically conducting material. The electrostatic discharge can create
locally a temperature up to 10,000 C. Both electrode and substrate material are immersed in
a dielectric fluid that also cools and removes debris from the processed location [95] . The electrode can
be machined with conventional techniques. Electropolishing usually follows EDM to both improve the
surface finish and remove the heat-affected zone.
An alternative for EDM is electro-chemical machining (ECM) [96] . This technique is based on the
electrochemical reaction between an electrode and a workpiece. The advantages of ECM are the low
mechanical stress and nonexistence of heat-affected zone as well as tool wear. The technique utilizing
both electro-discharge and chemical reaction for machining nonconducting materials is called spark-
assisted chemical engraving (SACE). In this technique, the substrate material does not work as an
electrode. The external tool electrode and counter-electrode work as the cathode and the anode,
respectively. The tool electrode is placed on the substrate surface and submerged in an electrolyte
solution (typically, sodium hydroxide or potassium hydroxide). At first, gas bubbles are generated at
the tool electrode due to electrolysis. If the voltage between the two electrodes is higher than a critical
value, the gas bubbles coalesce into a gas film isolating the tool electrode from the electrolyte. At this
moment, electrical discharges occur. The high temperature and probably chemical etching contribute
to the eroding of the nonconducting substrate placed next to the electrode. More details about SACE
are given in the recent review by W ¨ thrich and Fascio [97] .
4.3.4 Focused ion beam micromachining
Focused ion beam (FIB) micromachining uses highly focused ion beams such as Ga þ beam to scan and
cut the substrate surface inside a vacuum chamber. This technique was originally developed for sample
preparation in electron microscopy. The spot size of FIB is less than 10 nm. The removal rates can
further be improved by introducing reactive halogen gases into the processing chamber. Several effects
result from the ion bombardment. First, neutral and ionized atoms are removed from the substrate,
enabling micromachining of the substrate. The bombardment results in electron emission, which also
allows imaging of the sample. Furthermore, the ion beam can induce damages due to the displacement
of atoms and heating in the substrate. Chemical interactions, such as breaking of chemical bonds, can
be used for deposition. For more details on concepts and applications of FIB micromachining, readers
may refer to the recent review by Reyntjens and Puers [98] .
4.3.5 Powder blasting
Powder blasting is an erosion technique that uses kinetic energy of powder particles to generate cracks
on the substrate surface and consequently to remove material. The major process parameters of this
technique are particle material, particle size, particle velocity, and incident angle [101] . The technique
was originally developed for metals and further extended to silicon and glass. The main advantage of