Studies of the adhesion properties of LIGA microstructures by X-ray spectroscopy and mechanical measurements

In the first step of the LIGA process microstructures with high aspect ratios are fabricated by patterning a resist layer with deep etch X-ray lithography. As resist typically PMMA (polymethylmethacrylate) is used. For the following electroforming process and to achieve perfect microstructures the adhesion of the PMMA on the substrates is of great importance. For a better understanding the adhesion mechanism, particular induced through the adhesion promoter MEMO (methacryloxypropyl-trimethoxysilane) was investigated on various substrates. Two methods, namely XANES/EXAFS spectroscopy under grazing incidence and shear stress measurements were modified for the specific needs of microstructures. Our studies proved that the adhesion strength is determined by two factors. A rough surface, which allows mechanical interlocking, increases the adhesion strength by about 10%. The larger part of the adhesion strength is determined by formation of chemical bonds when adding an adhesion promoter. E.g. addition of 5% adhesion promoter increases the adhesion strength by 90%. These results were, as far as possible, confirmed by the X-ray spectroscopical measurements.