Characteristics of commercial PMMA sheets used in the fabrication of extreme high-aspect-ratio microstructures

We describe the characterization of various commercial brands of poly(methyl methacrylate) (PMMA) sheets used in tbe LIGA process, LIGA, a German acronym for lithography, electroplating, and molding, is a relatively new method for construction of extreme high-aspect-ratio microstructures. Metal structures with millimeter-by-micron dimensions can be fabricated by electrode-position of metals into developed features of the commonly used X-ray-sensitive resist, PMMA. Previous experiments from these laboratories have indicated that the adhesive strength of a given PMMA sheet to an electroplating substrate is a function of the source of the PMMA, thermal treatment of the PMMA, and the face of the PMMA sheet. We have used scanning force microscopy to inspect the topography of the faces of PMMA routinely used in the LIGA process. Depending on the thickness of the PMMA, or what face of a given PMMA sheet is inspected, the surface topography is found to vary from almost completely smooth to that where pits populate the surface. The dimensions and number of the pits depend on the thermal treatment of the PMMA sheets (annealing at an elevated temperature). Gas chromatography-mass spectrometry indicates that residual methyl methacrylate (MMA) monomer content in the PMMA decreases upon annealing. Sampling of the local atmosphere during annealing through use of solid phase microextraction confirms that residual MMA is released. The variation in pit number and size, the result of different amounts of expelled MMA, leads to the noted differences in adhesive strength of the various PMMA sheets to the substrate. (C) 1999 The Electrochemical Society. S0013-4651(98)10-009-5. All rights reserved.