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U. Schmid, A. Hajian, A. Bittner:
"Wet chemical Etching as an Effective Technique for the Local Permittivity Reduction of LTCC Substrates";
Vortrag: 15th International Conference and Exhibition on Ceramic Interconnect and Ceramic Microsystems Technologies (CICMT 2019), Shanghai, China (eingeladen); 16.04.2019 - 19.04.2019; in: "15th International Conference and Exhibition on Ceramic Interconnect and Ceramic Microsystems Technologies (CICMT 2019)", IMAPS, (2019), S. 1.

Rapid progress in the development of high-frequency (HF) devices increases also the specific requirements for the corresponding substrates. Because of their moderate permittivity (ɛr ~ 7...8) compared to alumina and their high integration level making use of the 3rd dimension, LTCC (low temperature co-fired ceramics) substrates have attracted much attention in the last two decades. Application scenarios such as radar sensors utilizing frequencies above 70 GHz require even better performance than commercial LTCC tapes can provide. A promising approach to improve the dielectric performance of glass-ceramic substrates is to modify the commercially available LTCC substrate locally by a wet chemical etching procedure. In [1] a novel approach is introduced which reduces the permittivity of LTCC substrates due to the generation of a tailored porosity in the glass-ceramic body utilizing the selective etching behaviour of different bulk-related crystallographic phases.
It is objective of the present study to provide an overview on these activities done in the last years as well as report on actual results such as the impact of sintering temperature on the porosification behaviour of LTCC. Basically, phosphoric acid, well known as an etchant for aluminium and alumina-based thin films, is used to attack the Al2O3 grains implemented in the glass matrix. But, to penetrate into the LTCC body up to a depth of about 90 μm, part of the glass-matrix needs to be dissolved as well. Besides glass and alumina being basic components in the green tape, crystalline anorthite or celsian are also generated during liquid-phase sintering by partial crystallisation of the glass phase through the diffusion of aluminium from the alumina filler into the enveloping glass matrix. Due to this process, a higher concentration of anorthite crystallizes from the glass phase when the sintering temperature is increased.
For a given tape composition fired with a certain temperature profile, the LTCC porosification behavior strongly depends on the chemical composition of the etching solution and its physical properties including concentration, and temperature. Very recently we have reported that the use of strong alkaline solutions as etchants offers the potential for a better surface quality with a suitable bearing plane for the realization of metallization lines [2, 3].
The resulting HF performance is measured by transmission measurements of ring resonators in the regime up to 70 GHz. By these measurements compared with high frequency simulations, a remarkable reduction of permittivity is demonstrated [4].