Photosensitive sol-gel precursors for direct ultra-violet patterning of SiO₂ and PZT materials

Abstract
The classical sol-gel method based on alkoxides hydrolysis and polycondensation has been modified in order to make photosensitive precursors suitable for direct patterning under U.V radiation after the spin coating. This avoids the photoresist masking and HF-HCl or RIE etching steps usually necessary for patterning the fired materia1.These photo-polymerisable solutions simplify greatly the selectivity problems in regards to other materials during etching and can also be used as a sacrificial layer for microelectromechanical systems (MEMS) processing (surface micromachining). The method was based on the addition of molecules that can polymerize under U.V in the sol-gel system and on finding a solvent compatible with both these molecules and the alkoxide system. Stability diagrams of these new solutions were made and the solutions were characterized by I.R and U.V spectroscopy. Photosensitive spin-on-glass of SiO₂ (P-SOG) and of ferroelectric PZT (P-FSOG) precursors were prepared using this technique and deposited on silicon wafers with different electrodes systems. Patterning was possible using a classical 365nm lamp on a contact lithography masker (Karl Suss). The non exposed regions were dissolved (negative resist) and the usual sol-gel process was then applied. SEM and optical photographs of 2D and 3D patterned structures show the spatial resolution that can be obtained by this technique which cannot be used for high density VLSI devices (FERAM, DRAM... ) but certainly for mesoelectronic systems of moderate integration scale like acoustic or pyroelectric sensors or MEMS.