Sol gel thin films on LTCC ceramic multilayers enable their use as thin film substrates

The sol gel process is an interesting method to functionalize different surfaces. It enables the deposition of non-metallic inorganic layers from the liquid phase. The sol is prepared from precursors, typically based on metal alkoxides. It is applied on the surface, where by dip coating, spray coating or spin coating can be used. The liquid sol flows into hollows and thus smooth the coarse ceramic. The gel-like layer contains liquid and solid portions. This layer is dehumified during the drying process and in a firing step a further densification an cross-linking is achieved. The firing temperature amounts to of 500°C and thus the sol gel can be classified as a low-temperature process, which enables cost-effective coating of layers with controlled properties. In this work two coating agents are compared: one alkoxide based on TEOS and one water based sol gel blend, filled with fused silica nanoparticles. The use of the silica nanoparticle dispersion enables layers with a thickness of 1 μm and thicker. Thus, the levelling of even rough surfaces is achieved with the method. The coating with pure TEOS leads to thinner layers. Surface qualities with optical quality are achievable. Beside of the coating process and the layer properties, the processing in thin film equipment is evaluated. The limits of lithography resolution are qualified and discussed. Finally, an outlook on possible applications is given.