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A. Hajian, St. Smetaczek, Ch. Zellner, M. Stöger-Pollach, T. Konegger, A. Limbeck, U. Schmid:
"Tailored and deep porosification of LTCC substrates with phosphoric acid";
Journal of the European Ceramic Society, 39 (2019), 3112 - 3119.

Low temperature co-fired ceramics (LTCC) as an advanced technology for robust assembly of electronic components,
has attracted significant attention in a wide application range such as in wireless communication or
automotive radar systems. However, accurate designs of micromachined devices operated at high frequencies
require substrates with regions of tailored permittivities. Introduction of controlled porosity into the substrate
via wet-chemical etching procedure, is a promising approach for permittivity reduction which can be applied to
commercially available LTCC without necessitating to alter their composition or sintering process. In the present
study, by selective dissolution of celsian phase a very deep porosification (highest reported so far) could be
realized while preserving the surface quality. Also, by a careful selection of the etching parameters, the depth of
porosification and hence the permittivity reduction can be delicately tailored. Laser ablation inductively coupled
plasma mass spectrometry was used for the investigation of chemical compositions of substrates.

http://dx.doi.org/10.1016/j.jeurceramsoc.2019.04.026