Fabrication of advanced LTCC structures for microwave devices

Abstract The main objective of this thesis was to research the integration of novel materials and fabrication processes into Low Temperature Co-fired Ceramic (LTCC) technology; enabling fabrication of Radio Frequency (RF) and microwave components with advanced performance. The research focuses on two specific integration cases, which divide the thesis into two sections: the integration of tunable dielectric structures and the integration of air filled waveguides. The first section of the thesis describes the development and characterization of low sintering temperature Barium Strontium Titanate (BST) thick film paste. Sintering temperature of BST is decreased from approximately 1350 °C down to 900 °C by lithium doping and pre-reaction of the doped composition. This allows the co-sintering of the developed BST paste with commercial LTCC materials. Additionally two integration techniques to embed tunable components in an LTCC substrate using the developed BST paste are also presented and the electrical performance of the components is evaluated. The highest measured tunability value was 44% with a bias field of 5.7 V/µm. The permittivity of the films varied between 790 and 190, and the loss tangent varied between 0.004 and 0.005, all measured unbiased at 10 kHz. The developed LTCC compatible BST paste and the presented integration techniques for tunable components have not been previously published. In the second section of the thesis, a fabrication method for the LTCC integrated air-filled rectangular waveguides with solid metallic walls is presented. The fabrication method is described in detail and implemented in a set of waveguides used for characterization. A total loss of 0.1–0.2 dB/mm was measured over a frequency band of 140–200 GHz. The electrical performance of the waveguides is evaluated and their use demonstrated in an integrated LTCC antenna operating at 160 GHz