Design and Fabrication of Ultra-Wideband Bandpass Filter Based on Liquid Crystal Polymer Substrates

Licentiatavhandling, 2008

For millimeter (mm)-wave systems’ applications, as frequency is moving towards higher and higher bands, materials and integration technologies are witnessing higher performance demands and consequently higher constraints. From a material’s point of view, the selection of substrate material has recently drawn considerable attention in the design of the whole mm-wave system. The materials presently used as mm-wave substrates are either expensive or show unsatisfactory performance. Indentified as advanced candidate materials for flexible mm-wave substrate purposes and due to a unique combination of superior features and performance, Liquid Crystal Polymers (LCPs) have attracted considerable attention in commercial wireless applications. Like other materials, LCPs also show some disadvantages such as poor adhesion to Copper (Cu), problems in via processing, etc. Since the U.S. Federal Communications Commission (FCC), in 2002, released the unlicensed use of Ultra-Wide Band (UWB) systems for different applications (3.1-10.6 GHz band for the indoor and hand-held systems and the 22-29 GHz band for automotive radar systems), much interest and many research activities have been aroused on exploring a variety of UWB components and devices. Of them, the UWB bandpass filter has been identified as one of the key circuit blocks in the whole UWB system. Although much effort has been invested in experimental and theoretical studies of planar microstrip UWB filters, it is still difficult to get filters with excellent out-of-band and in-band performance and simultaneously low cost and small area. Much work has been both performed and reported on UWB filters, working in the 3.1-10.6 GHz range, however, only a very limited amount of research papers are available on millimeter-wave UWB filters working over the 22-29 GHz band. In this thesis, and by combining LCP substrates and UWB technology, a new and compact microstrip UWB bandpass filter, working over a frequency range of 22-29 GHz, is realized for automotive radar systems’ applications. Based on the dual-mode resonator theory, a UWB bandpass filter, with a center frequency of 25.5 GHz and a bandwidth of 7 GHz, is proposed. The proposed UWB bandpass filter includes a full dual-mode rectangular-ring resonator at the center, which controls the bandwidth of the passband and two novel parallel-coupled feed structures between input/output ports and the ring resonator, for coupling purposes with the resonator. The planar microstrip filter is fabricated onto the LCP substrate using standard processing methods i.e. sputter deposition, photolithography and wet etching. In order to improve the surface roughness of the LCP substrate, the surface is pre-treated by means of oxygen plasma etching. In order to improve the adhesion between the Cu and the LCP material, prior to the sputter deposition of Cu, a thin layer of Titanium (Ti) is applied. Furthermore, this work also includes the fabrication on LCP of a planar wideband bandpass filter working over 50-70 GHz band. The comparison of simulated and measured results of both developed bandpass filters is also shown and discussed. Both bandpass filters show good results, proving that LCP materials are potential substrate material for mm-wave applications.