LTCC-Strukturen für Hochfrequenzanwendungen

Abstract

Low Temperature Co-fired Ceramics (LTCC) technology is mainly used in electronics and microfluidics. To accomplish a whole structure several thin layers of the ceramic material are combined and fired in a furnace. Before that, the single layers may be shaped nearly arbitrarily. Furthermore, additional materials may be applied on each layer using thick-film technology or photochemical processes. Therefore, it is possible to design complex structures and build electrical components like conducting tracks, capacitors, inductors, and resistors embedded into the structure. The unfired ceramic tapes, also called green sheets, are shaped independently first. Then they are stacked and laminated. Finally, the whole structure is fired in a furnace with a well defined temperature profile peaking at about 850 - 900 °C. This process combines the advantages of HTCC (High Temperature Co-fired Ceramics) and thick-film technologies, and is an affordable option to conventional circuit board technologies when dealing with small and middle range number of units. There are many different kinds of ceramic substrates with different relative permittivities available on the market, which may be combined within one structure as long as they are matching. In addition, these ceramics provide very low dielectric losses at high frequencies and have an adequate good thermal conductivity compared to conventional used materials what may be a big advantage when dealing with power electronic applications. Since the TU Wien operates its own LTCC laboratory at the Institute of Sensor and Actuator Systems, this diploma thesis will focus on evaluating the possibility to produce high frequency structures with the given environment. The given frequency range was 500 MHz up to 10 GHz. To take advantage of the benefits explained before, the possibility of building passive components into the ceramic structures was examined. First it was evaluated which kind of transmission lines is best suitable for this task. Afterwards, the geometries of passive components like capacitors were determined empirically using field simulation to achieve good high frequency characteristics. For the LTCC - manufacturing process empirical values from laboratory staff members were used mainly. However it was necessary to evaluate some parameters by myself and make some adaptations during the fabrication. Thick-film technology was used for the layers of the LTCC component, which was finally fired. Finally, the produced components were measured and compared to the simulation results. To achieve a range of capacity values, 16 different capacitors were constructed. They differed in the plate area and the number of plates. This resulted in a capacity range from about 0.15 pF up to 32 pF. The measured and simulated S-parameters of the capacitors showed a very good conformity up to the first resonance frequency. Between the first and the second resonance frequency a little deviation was observed, which especially occurred for the devices with larger plate areas. Since the interesting range of a capacitor is up to the first resonance frequency, this could be left out of consideration. Furthermore, the capacitors were compared regarding their different sizes and number of plates. It showed that the more compact devices provide better high frequency characteristics. Comparing two capacitors with the same value, one with a smaller plate area but a higher number of plates provides a higher first resonance frequency. In addition, equivalent circuits were found, which showed very good matches to the first as well as to the second resonance frequency. The LTCC technology showed good characteristics for high frequency applications. Especially the advantage of building 3D-structures and including conductor structures within the LTCC-boards provides more opportunities in circuit design. However, it has to be considered, that the manufacturing process requires plenty of time and has to be performed in an accurate and careful way. Also the shrinkage of the LTCC structures is a big disadvantage, since it results in an accuracy of the spatial dimensions within 0.2 %.