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LIGA and Micromolding 4-3

Idea Electroplate micromolds Plastics replication Bonding
Master fabrication Injection molding With adhesives
Design Seed layer Hot embossing With solvents
CAD Silicon bulk micromachining - CVD/PVD metal Plastics casting With heat/pressure
Photomask - Wet etching Electroplating
- Deep reactive ion etching - Over Si
Photodefinable glass - Over glass
Mechanical micromachining - Over thick resists
- Micro - EDM - Over bulk metal
- Excimer laser
- CNC machining
Lithography Deposition of Casting
- Deep UV
electrical
- X-ray exposure (LIGA) layer
E-beam machining
Ion-beam machining
Electroplating
Pressure
hot
Silicon bulk Backside embossing
micromachining planarization

Heat
Mechanical Heat
micromachining Separation mold Pressure
insert from master
Injection
molding
Lithography

FIGURE 4.2 Process flow for plastic microfabrication.

Micromachining techniques are reshaping manufacturing approaches for a wide variety of small parts.
Frequently, IC-based batch microfabrication methods are considered along with more traditional serial
machining methods. In this evolution, LIGA and pseudo-LIGA processes constitute “handshake-
technologies” bridging IC and classical manufacturing technologies. The capacity of LIGA and pseudo-
LIGA for creating a wide variety of shapes from different materials makes these methods akin to classical
machining, with the added benefit of unprecedented aspect ratios and absolute tolerances rendered
possible by lithography and other high-precision mold fabrication techniques.
In this chapter, after a historical introduction to LIGA, we will analyze the process steps depicted in
Figures 4.1 and 4.2. We start with a description of the different applications and technical characteristics
of synchrotron radiation and then present an introduction to the crucial issues involved in making X-ray
masks optimized for LIGA. Emphasis is on the two most important additive steps in LIGA and LIGA-like
methods: electro- and electroless deposition of metals and plastic micromolding. Micromolding with an
elastomeric mold, one of the many types of soft lithography, is widely used in research due to its low cost
and easy implementation. Bonding of plastic molded micro parts, including plastic welding, organic
solvent bonding, and bonding with thermoset adhesives, is covered in Madou (2002).

4.2 LIGA — Background

4.2.1 History

LIGA combines the sacrificial wax molding method, known since the time of the Egyptians, with X-ray
lithography and electrodeposition. Combining electrodeposition and X-ray lithography was first carried
out by Romankiw and co-workers at IBM as early as 1975 [Spiller et al., 1976]. These authors made high-
aspect-ratio metal structures by plating gold in X-ray-defined resist patterns of up to 20 µm thick. They
had, in other words, already invented “LIG”; that is, LIGA without the abformung (molding) [Spiller et al.,

© 2006 by Taylor & Francis Group, LLC