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|Investigation on the Porosification Behaviour of Ffired LTCC Substrates|
|Keywords: LTCC, porosity, wet chemical etching|
|Rapid progress in the development of high frequency (HF) devices increases also the specific requirements for the corresponding substrates. LTCC (low temperature co-fired ceramics) has attracted much attention as high frequency substrates because of its moderate permittivity (~ 7..8) compared to alumina substrates and because of its high integration level making use of the 3rd dimension. 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 ceramic substrate locally by a wet chemical etching procedure. In  a novel approach is described 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 phases. It is objective of the present work to investigate the impact of sinter temperature on the microstructure of the LTCC and hence, to study the porosification procedure in more detail. Phosphoric acid, well known as 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 about a depth of 40 µm, part of the glass-matrix needs to be attacked in addition to the Al2O3 grains. Besides glass and alumina being basic components in the green tape, crystalline anorthite is generated during liquid-phase sintering by partial crystallisation of the glass phase, which is supported by the dissolution of alumina in the glass matrix.. Due to this process, a higher concentration of anorthite crystallizes from the glass phase when the sintering temperature is increased, which is proved by TEM analyses. The etching of anorthite in phosphoric acid is demonstrated by means of µ-XRD measurements. Furthermore, the etchant is chemically analyzed before and after exposure to LTCC utilizing ICP-MS technique. Besides the anorthite, a high concentration of Pb in the etchant indicates a partial attack of the glass-matrix. In the final paper we will present detailed results of the etching behaviour of LTCC “DP 951”.  A.Bittner, U. Schmid: The porosification of fired LTCC substrates by applying a wet chemical etching procedure, J. Europ. Ceram. Soc., 2009, Vol. 29, pp. 99-104.|
|Achim Bittner, Ph.D. Student
Saarland University, Chair of Micromechanics, Microfluidics/Microactuators
Saarbruecken, Saarland 66123,