Title: Wet chemical porosification of LTCC substrates for high frequency and catalyst support applications
Language: English
Authors: Hajian, Ali 
Qualification level: Doctoral
Advisor: Schmid, Ulrich 
Issue Date: 2020
Number of Pages: 216
Qualification level: Doctoral
Abstract: 
Nowadays, due to the continuously increasing demand for larger bandwidths and data rates, there is a growing interest in the use of microwave and millimetre wave frequencies in various applications such as mobile phones, automotive radar systems, and wireless communication systems. Low-temperature co-fired ceramics (LTCC) which are advanced composites of glass and ceramicssintered at temperatures below 1000 °C, are capable to fulfil these demands. Due to several exceptional features in the field of dielectric and thermomechanical properties, especially highmechanical fracture strength, low weight, high integration level, and compatibility with radio frequency microelectromechanical systems (RF-MEMS) and monolithic microwave integrated circuits (MMICs), LTCC technology has attracted significant attention for the fabrication of high-density multilayer packages. However, advanced designs of micromachined devices operating at high frequencies require substrates with regions of tailored permittivities. Wet chemical etching with an appropriate etching solution under defined conditions leads to channel-like, statistically distributed and interconnected open meso- to macro-pores which cause a reduction in permittivity of commercially available LTCC tapes without the need to alter the tape composition or the firing process.The main challenges associated with this approach, however, are achieving a high degree of porosification, thus increasing air embedment while keeping the surface as intact as possible. To ensure that the high-frequency devices can work at maximum efficiency, the surface topography of the LTCC needs to be preserved, since a highly destructed surface prohibits further metallization lines. Additionally, by increasing frequencies up to the GHz range, the skin depth, derived for ideally smooth conductor surfaces, decreases to the order of the surface roughness, thus causing an early linear increase in conductor loss. Moreover, the etching may lead to other changes in the physical properties of the LTCC, including detrimental effects on mechanical properties. Therefore, this thesis mainly aims towards the introduction of an alternative approach for porosification of the LTCC surface targeting a better surface quality which features a suitable bearing plane for further metallization lines without the need for wire bonding. Therefore, the etching behaviour of commercially available LTCC tapes was investigated with different etching solutions and at different etching conditions, i.e., etchant temperature, pH, and concentration. Remarkable results were obtained under defined conditions where for the first time, a very deep porosification of the whole LTCC substrate while preserving the surface quality and the original substrate thickness could be obtained. Besides, the impact of firing temperature on morphology, phase composition, as well as porosification was studied. The effective relative permittivity of LTCC substrates at 1 GHz was reduced up to 10.8% when measuring substrates with a thickness of approximately 600μm and a porosification depth up to 186 μm from each side. The calculated relative permittivity values for the etched layer showed a reduction of up to 22% in comparison to the initial value for the as-fired LTCC. To explore the impact of porosification on the mechanical properties of LTCC substrates their stiffness behaviour after wet-chemical etching was investigated using dynamic-mechanical analysis at temperatures up to 550 °C. Promising results were obtained which demonstrate the applicability of such modified modules even when operated at such high temperatures. Moreover, a miniaturized biaxial bending test called Ball On Three Balls test (B3B) was developed and successfully used for the flexural strength measurement of different LTCC tapes. The developed setup can be used for testing other brittle materials such as pure ceramics. Finally, the potential of porosified LTCC for green energy production was explored. For this purpose, first porosified Ferro L8 LTCC substrates were impregnated with palladium nanoparticles. The developed catalyst was then successfully tested for hydrogen production from methanol. The obtained results were promising and indicate the potential of porosified LTCC for application in fuel cells and microreactors. However, additional investigations need to be performed to improve the catalyst performance.
Keywords: Low temperature co-fired ceramics; Wet-chemical etching; Catalyst support; Permittivity reduction; Mechanical properties
URI: https://doi.org/10.34726/hss.2020.84520
http://hdl.handle.net/20.500.12708/16051
DOI: 10.34726/hss.2020.84520
Library ID: AC16061166
Organisation: E366 - Institut für Sensor- und Aktuatorsysteme 
Publication Type: Thesis
Hochschulschrift
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