Spinodal decomposing glass ceramic with tailored porous, thermal and dielectric properties
Final Report Abstract
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. By selection of the optimum etchant composition and etching conditions through systematic investigations on commercially available LTCC, 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 550 °C. 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. Alternatively new LTCC assemblage were developed. A LTCC is constituted by two types of material, a filler giving the required properties and a binder having a low melting point, generally a glass, generating a smooth surface and assuring the coherence of the final ceramic. The original idea of this project was to play on the nature of both the binder and the filler to maximize the chemical dissolution and preserve the surface and mechanical integrity. This can be reached using a glass with the ability to spinodal decomposition. In that case the glass separates in two interconnecting 3D networks between a silica rich and a silica poor phase. The wet chemical etching acts preferentially on the silica poor phase. The reminding silica rich phase warranty good mechanical properties. Five different binder/filler combinations were identified for their etching ability and electrical properties. All production parameters of these new LTCCs were studied and green tapes casted and sintered with success. For all tapes the relative permittivity decrease by a factor close to 3 after etching and the dissipation factor by a factor 10. The surfaces remained enough unaffected to allow future metallization. Throughout this project, specific characterization methods of the materials were developed, in particular the determination of the extent of the etched zones, the monitoring of phase separation, the special repartition of the reminding phases using Raman and Brillouin vibrational spectroscopies and advance scanning electron microscopy. Currently, porosified LTCC substrates are measured with respect to their dielectric properties up to the GHz range.
Publications
- “Porosification Behaviour of LTCC Substrates with Potassium Hydroxide” Journal of the European Ceramic Society, 38, (2018), 2369-2377
A. Hajian, M. Stöger-Pollach, M. Schneider, D. Müftüoglu, F.K. Crunwell, U. Schmid
(See online at https://doi.org/10.1016/j.jeurceramsoc.2018.01.017) - “On the porosification of LTCC substrates with sodium hydroxide” Composites Part B: Engineering, 157, (2019), 14-23
A. Hajian, D. Müftüoglu, T. Konegger, M. Schneider, U. Schmid
(See online at https://doi.org/10.1016/j.compositesb.2018.08.071) - “Tailored and deep porosification of LTCC substrates with phosphoric acid” Journal of the European Ceramic Society, 39, (2019), 3112-3119
A. Hajian, S. Smetaczek, C. Zellner, M. Stöger-Pollach, T. Konegger, A. Limbeck, U. Schmid
(See online at https://doi.org/10.1016/j.jeurceramsoc.2019.04.026) - “Wetchemical porosification of LTCC substrates: Dissolution mechanism and mechanical properties” Microporous and Mesoporous Material, 288, (2019), 109593-109602
A. Hajian, M. Brehl, T. Koch, C. Zellner, S. Schwarz, T. Konegger, D. de Ligny, U. Schmid
(See online at https://doi.org/10.1016/j.micromeso.2019.109593)
