Manufacturing aspects of LIGA technologies

Abstract

Different concepts for the manufacture of masks for deep X-ray LIGA are presented. The variation in the accuracy of the final mask as a function of the mask manufacture concept (low, medium or high resolution) influences the sidewall roughness of the irradiated PMMA structures considerably. The mask membrane also influences structure quality. Graphite masks, which yield PMMA resist structures with straight sidewalls, have increased surface roughnesses (>2 μm Rz) comopared to beryllium. A design of experiments approach to the optimisation of PMMA for LIGA applications with deep X-ray lithography has been undertaken. For the optimisation, 18 different resist mixtures were produced on the basis of the significant parameters, holding the parameters within values that were experimentally shown to be conducive for PMMA resist production. High ratios of crosslinker in the resist mixture was shown to lead to significant amounts of resist residue between the microstructures; resist development was significantly hindered. Other resists with higher ratios of adhesion promotor in the resist also had significant problems with development. PMMA resist mixtures containing between 2% and 5% adhesion promotor led to microstructures with sufficient adhesion to the substrate surface without any substantial problems with the development step. A variant of the LIGA-process, which involves deep silicon etching for mastering, has successfully been applied for the fabrication of disposable plastic chips for 2-dimensional electrophoresis. The high precision and structure accuracy of the replicated plastic devices as well as the possibility to pattern complex multilevel structures demonstrate the huge potential of LIGA based on silicon micromachining for the realization of microfluidic and biomedical devices.