The porosification of fired LTCC substrates by applying a wet chemical etching procedure

In this study, a novel process is presented to generate a defined and homogeneous degree of porosity in fired low temperature co-fired ceramics (LTCC) substrates. For this purpose, a phosphoric-based acid is used which is a standard wet chemical etchant in the MEMS and microelectronic industry for the patterning of aluminium-based conductors and strip lines. Varying the bath temperature between 90 and 130 °C within a time frame of up to 8 h, a maximum penetration depth of 40 μm is achieved. At short etch times up to 5 h, the porosification process is reaction controlled, while at longer exposure times, diffusion-related effects dominate verified by the determination of the corresponding activation energies. In combination with morphological investigations using scanning electron microscopy and micro-X-ray diffraction techniques, it is demonstrated that the anorthite-phase crystallizing during liquid sintering in the vicinity of the Al2O3 grains shows a high dissolvability in phosphoric acid and is very important to enable its penetration into the LTCC body. This surface-near process is very attractive for the realization of selected areas on conventional LTCC substrates having modified dielectric properties, especially for high frequency applications.