Chapter 5: The LIGA Microfabrication Technique

1 Overview

The production of microstructures by the LIGA process is schematically represented in Fig. 1 and 2. The essential process steps are X-ray lithography with synchrotron radiation, the electroplating of metals and the molding of plastics. This combination of process steps have been given the name LIGA; LI for X-ray lithography, G for galvanic or electroplating and A for Abformung (German word for molding). In X-ray lithography, as a first step, a plastic layer several hundreds of micrometers thick is applied to a metallic base plate or an isolated plate with an electrically conductive cover layer used as the substrate. The X-ray sensitive plastic is either polymerized in place directly on the base plate or glued to it. Until now PMMA (polymethyl methacrylate) has been used almost exclusively as X-ray resist because of its high contrast known from electron beam lithography. To form the micro structure an absorber pattern of a mask is transferred into the thick plastic layer with the aid of extremely parallel and high intensive synchrotron radiation with a characteristic wavelength between 0.2 and 0.6 nm. The X-ray radiation which passes through the mask is absorbed in the resist and leads to a chemical modification. In the case of PMMA the chemical resistance changes because of bond breaking in the long molecular chain, so that these regions can be dissolved with a...