Spinodal decomposing glass ceramic with tailored porous, thermal & dielectric properties

Within the project a novel glass-ceramic material will be developed providing a nanoscale porosity after a wet chemical leaching process. The material composition will be composed so that while a temperature treatment a spinodal decomposition occurs and an additional leachable phase originates. This material will be manufactured by a specific tape casting process. For that, the original material composition is conditioned as slurry and cast to a defined sheet thickness. The shape and size of the decomposed phases depends mainly on the annealing steps. By this procedure a homogeneous distribution of the occurring phase can be achieved. There are some suitable glasses for this process being investigated in Erlangen. After sintering the decomposed phase will be wet chemically etched and the degree of porosity can be adjusted. Depending on the porosity and the pore size a suitable thin film metallization will be investigated covering the pores and being suitable e.g. as heater element. For that, the film will be investigated with respect to its electrical performance and long term stability. As demonstrator, a Pirani pressure sensor will be manufactured, showing the low thermal conductivity and suitability of these materials for the applicability within sensors. Additionally, the permittivity of the material will be investigated as further interesting property of porous dielectric materials.

Low temperature co-fired ceramics (LTCC) is an advanced substrate technology for the robust assembly and packaging of electronic components for micromachined devices. Due to the multilayer approach based on glass-ceramics sheets, this technology offers the possibility to integrate passive electrical components such as capacitors, resistors, inductors, and conductor lines into the LTCC body. Due to its high robustness and reliability, it is a very attractive substrate technology for a wide application range such as in wireless communication or in automotive radar systems. However, accurate designs of micromachined devices operated at high frequencies require substrates with regions having a tailored permittivity in one single layer. These areas with low permittivity enhance both the bandwidth and the efficiency of the active components while the high permittivity areas allow a compact feeding circuit design. Therefore, a locally controlled variation of the permittivity by introducing areas of different permittivity in one single layer LTCC is of great importance. Applying a wet-chemical etching process to the accurately masked LTCC substrates is the state-of-the-art approach which allows a local permittivity reduction. This method is essentially based on a local air embedment in the surface-near regions of the LTCC through a wet-chemical etching process and thereby replacing high permittivity components of the LTCC substrate with air having a low permittivity. The main concerns associated with this method are the increased roughness which is challenging the further metallization, and the limited depth of porosification which results in less air embedment and hence, in a low permittivity reduction. Most recently, by selection of the optimum etchant composition and etching conditions through systematic investigations, we achieved a deep and tailored porosification depth while preserving the original surface quality. However, the very high depth of porosification may raise concerns about the mechanical robustness of the modified LTCC. Therefore, using temperature-dependent dynamic-mechanical analysis, the stiffness behavior of the LTCC substrates after wet-chemical etching was investigated, and promising results for the applicability of such modified modules were obtained, even when operated at elevated temperatures up to 550C. Also, a practical correlation between the mechanical properties and the relative porosification depth was introduced, which is independent of etching conditions and the substrate thickness, and is valuable for optimization of the suitable depth of porosification for securing the desired mechanical properties. In addition we showed that under defined conditions, in addition to generating a tailored porosity, by partial dissolution of the LTCC surface its overall thickness reduction was also possible. Currently, porosified LTCC substrates are measured with respect to their dielectric properties up to the GHz range.