Integrated microwave structures using an advanced thick-film technology

The rapid expansion of wireless technology for commercial purposes, notably mobile telephony, has created a demand for low cost, high quality, fabrication processes and materials capable of use for microwave work. In this paper a thick-film process is described which not only meets the current requirements for wireless technology, but has been shown to offer outstanding potential for advanced integrated structures operating at higher microwave frequencies, into the mm-wave band. In addition, the paper addresses the key issues of material characterisation at microwave frequencies and describes some new techniques for obtaining accurate data on relative permittivity and loss tangent of thin dielectrics. Using this technique, measurements have been made on a new low loss low temperature co-fired ceramic (LTCC) material and results are reported. Measurements have also been made to establish the principal electrical properties of the material, namely dielectric loss, dielectric permittivity and line loss. The key measured data are: (i) very low loss tangent, 10^-4 over the frequency range 8-12 GHz, (ii) consistent values of 3.9 for the dielectric relative permittivity, (iii) low line loss to at least 40 GHz, and (iv) low dispersion. Experimental work has also been carried out to compare the relative performance of etched and printed thick-film lines, using a range of substrates, including LTCC materials, over a wide range of frequencies. In particular, measured data on 50 ohm microstrip lines fabricated on alumina using the new thick-film gold technology gave line loss values varying from 0.005 dB/mm at 2 GHz to 0.033 dB/mm at 40 GHz, which is a further indication of the performance that is obtainable from the technology