The LIGA Fabrication process provides the possibility to produce micromechanical structures with very high aspect ratios compared to other microelectromechanical technologies (up to 300:1). The height of the manufactured pieces can be 100 microns to a couple of millimeters. By using molding, different materials (metal, ceramics as well as plastic) can be used to produce the final structure.
LIGA is a German abbreviation and stands for Lithographie, Galvanoformung and Abformung (lithography, electrodeposition and molding). The process was developed in the 1980s by the Kernforschungszentrum Karlsruhe, Germany (KfK) to provide a technique for producing large numbers of micron sized nozzles, and is now mainly pursued by two German companies, Microparts (Dortmund) and the Institut für Mikrotechnologie Mainz GmbH (IMM).
The principle of the LIGA process consists of depositing a relatively thick layer of a polymer sensitive to X-rays on top of a conductive substrate or one covered with a conductive seed layer. This can be done by applying multiple coats of photoresist during spinning the substrate wafer for thicknesses of up to a few hundred microns. For thicker polymer layers (millimeters) it is common to buy prefabricated plates of PMMA (polymethylmethacrylate, a polymer often used as photoresist), attach them to the substrate plate and mill them back to the desired thickness.
After exposure through an appropriate X-ray-mask, a developer removes either the exposed (positive photoresist) or the unexposed (negative) areas of polymer, and metal layers are grown by electroplating in the spaces now free from cover. Often the metal is grown higher than needed and then milled back to the desired thickness together with the photoresist. Having removed the unwanted areas of the polymer, the resulting metallic structure can be used. Free mechanical structures can be manufactured by using a sacrificial layer between the wafer substrate and the grown metal film and dissolving this layer, thus yielding the structure in the end of the process; this technique is called sacrificial LIGA (SLIGA).
Figure 6: The LIGA process as done by IMM, Germany16
For the purpose of mass production, the metal can serve as a mould or embossing tool for pieces made of various materials such as metals, polymers, ceramics or glass.
The major advantages of the LIGA process are the high aspect ratios made possible and therefore the larger heights of the pieces, very sharp vertical sidewalls as well as the possibility to produce three-dimensional structures by using a sacrificial layer as described above.
The main disadvantage of LIGA is the need of high-energy X-rays that can only be achieved with a synchrotron. This and the expensive X-ray masks needed for exposure make the cost for the process high. On the other hand, the mass production of microstructures by using the produced structures as mold for other materials becomes very inexpensive per produced device, since the expensive exposure step only needs to be done once in the beginning of the fabrication process.
To reduce the cost of LIGA, especially the investment costs for the synchrotron, the usage of traditional lithography light sources in the UV range is being investigated in combination with other photoresist materials (rather polyamides or standard photoresists than the polymers used in X-ray lithography). Using these processes, structures of up to 80 microns in height can be produced with this technique.
Alternatively companies providing the necessary tools for LIGA to multiple users can reduce the production cost, too. MCNC in Research Triangle Park, NC offers LIGAMUMPs™, a "Multi-user MEMS Process that involves the fabrication of high aspect ratio MEMS in a low-cost environment"17.