LIGA Technologies and Applications
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LIGA Technologies and Applications Jill Hruby Introduction LIGA, an acronym for the German words for lithography, electroplating, and molding, is a technique used to produce microelectromechanical systems (MEMS) made from metals, ceramics, or plastics. The LIGA process utilizes x-ray synchrotron radiation as a lithographic light source. Highly collimated, high-energy x-rays from the synchrotron impinge on a patterned mask in proximity to an x-ray-sensitive photoresist, typically poly(methyl methacrylate) (PMMA). In most cases, the PMMA is attached to a substrate that is used later as an electroplating base. The regions of PMMA irradiated by the x-rays experience bond scission, and these areas are selectively dissolved in a chemical developer. Once the PMMA is developed, the resulting nonconductive mold is filled with metal by electrodeposition from a conductive base. As originally envisioned, after electrodeposition and dissolution of the PMMA, the electroplated structures can be removed from the substrate and used as individual metal microparts, or the metal pattern can be used as a mold insert for replication in a polymer using injection molding or hot embossing. LIGA parts typically have 1–10-m minimum lateral feature sizes, and are from hundreds of micrometers to a few millimeters thick. The format used for LIGA processing is generally 3–6-in.-diameter wafers. These processed wafers may contain thousands of microparts or a few larger parts with small, precise features. The LIGA process was first developed at the Forschungszentrum Karlsruhe (FZK) in the mid-1980s.1 Since that time, there has been considerable investment in process research, as well as technology maturation for commercial application, at institutions worldwide. A technique similar to LIGA that utilizes UV radiation instead of synchrotron radiation to produce patterns has also been developed and is usually referred to as UV-LIGA.2 More recently, several other techniques to produce LIGA-like structures without using synchrotron radiation have been developed, MRS BULLETIN/APRIL 2001
including deep reactive ion etching (DRIE)3 and laser ablation.4 These techniques use other methods to create a mold in a nonconductive polymer or a semiconducting material, which can then be electroplated and processed in a manner similar to x-ray synchrotron patterned wafers. Likewise, techniques other than polymeric replication have been developed to allow microstructures to be replicated from LIGA masters in other materials. This article reviews LIGA processing technologies and describes examples of some current applications using LIGA techniques.
LIGA Processing Technologies LIGA, like other microfabrication technologies, requires a number of serial processes to create a finished product. When a metal micropart is the desired product, the processing steps shown in Figure 1 are followed. When LIGA is used to produce a metal mold insert for replication, the processing steps after plating are modified (and discussed in more detail later). While optimizing the layout
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