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A. Hajian, D. Müftüoglu, T. Konegger, M. Schneider, U. Schmid:
"On the porosification of LTCC substrates with sodium hydroxide";
Composites - Part B: Engineering, 157 (2019), 14 - 23.

Outstanding material properties of low temperature co-fired ceramics (LTCC) make them the technology of
choice when targeting the realization of robust, highly integrated substrates or packaging solutions for micromachined
devices. However, the relatively high permittivity limits their utilization in high-frequency applications,
as in addition defined areas with reduced permittivity in one single LTCC layer would be most beneficial.
The wet-chemical porosification method under alkaline conditions is a novel and most advantageous approach
which can be applied for permittivity reduction through locally embedding air into the LTCC surface in its asfired
state with low impact on the surface characteristics. The resulting porosity of the LTCC substrates after the
etching process is strongly affected by the LTCC tape composition as well as the etch parameters. Since the
chemical composition of the tape is desired to be unaltered, optimizing the etching parameters, namely etchant
concentration, etching time, and bath temperature is a reasonable procedure which allows tailoring the porosification
process for the LTCC tapes in their as-fired state.
In this work the etching behavior and surface morphology of the LTCC substrates have been studied, both
qualitatively and quantitatively, and detailed gravimetric and roughness measurements, as well as porosification
depth investigations, were carried out. The conducted analyses suggest a dominating reaction-controlled mechanism
for the etching process. In addition to generating a tailored porosity in the surface-near region, overall
thickness reduction of the LTCC through the overall dissolution of the LTCC surface under a defined etching
condition was also possible. The experimental results were finally fitted into a nonlinear polynomial model for
providing an experimental basis in order to identify the most crucial parameters to achieve a tailored porosity.

http://dx.doi.org/10.1016/j.compositesb.2018.08.071