| 3.5 High-Aspect-Ratio Micromachining
High-aspect-ratio micromachining (HARM) is a process that involves micromachining as a
tooling step followed by injection moulding or embossing and, if required, by electroforming
to replicate microstructures in metal from moulded parts. It is one of the most attractive
technologies for replicating microstructures at a high performance-to-cost ratio and includes
techniques known as LIGA. Products micromachined with this technique include high-
aspect-ratio fluidic structures such as moulded nozzle plates for inkjet printing and
microchannel plates for disposable microtitreplates in medical diagnostic applications. The
materials that can be used are electroformable metals and plastics, including acrylate,
polycarbonate, polyimide and styrene.
3.5.1 LIGA
LIGA is an important tooling and replication method for high-aspect-ratio microstructures.
The technique employs X-ray synchrotron radiation to expose thick acrylic resist of PMMA
under a lithographic mask (see Figure 24 below). The exposed areas are chemically dissolved
and, in areas where the material is removed, metal is electroformed, thereby defining the tool
insert for the succeeding moulding step. LIGA is capable of creating very finely defined
microstructures up to 1000 µm high.
LIGA is limited by the need to have access to an X-ray synchrotron facility. A compromise
which combines some features of LIGA with surface micromachining eliminating the need
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