Design, Fabrication and Characterization of Microwave Passive Devices on Liquid Crystal Polymer Substrates

Doktorsavhandling, 2009

For microwave/millimeter (mm)-wave systems’ applications, as frequency is moving towards higher and higher bands, materials and integration technologies are witnessing higher performance demands and constraints. The materials presently used as mm-wave substrates are either expensive or show unsatisfactory performance. Identified as advanced candidate materials for flexible mm-wave substrates’ 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. In order to evaluate LCP’s electrical performance, the design, fabrication and characterization of passive components operating at frequencies extending into the mm-wave regime are performed to determine the feasibility of LCP as a low-cost substrate solution for mm-wave application. Since the U.S. Federal Communications Commission (FCC) released the unlicensed use of Ultra-Wide Band (UWB) systems for different applications, much interest and many research activities have been aroused on exploring a variety of UWB components and devices. Of them, UWB bandpass filter and UWB antenna have been identified as two of the key circuit blocks in the whole UWB system. Although much effort has been invested in experimental and theoretical studies of microstrip UWB filters and UWB antennas, it is still difficult to get filters with excellent performance and simultaneously low cost and compact size, and wide bandwidth antennas with small area. In this thesis, two novel compact microstrip UWB bandpass filters, an UWB printed monopole antenna and a 60 GHz parallel-coupled line bandpass filter are presented on LCP substrates. Based on the dual-mode ring resonator theory, two UWB bandpass filters, working on 22-29 GHz and 50-70 GHz band, are 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. Moreover, a tapered Co-Planar Waveguide (CPW)-fed hollowed UWB printed monopole antenna is also presented on a LCP substrate. It consists of a hollowed elliptical monopole, two trapeziform ground planes and a tapered CPW feeder, which provides this antenna extremely wide bandwidth and compact size. Finally, a 60 GHz band planar microstrip bandpass filter with ultra-fine conductor linewidth and gap is reported based on the parallel-coupled line structure. By using standard processing methods i.e. surface pretreatment, sputter deposition, photolithography and wet etching, all of these microwave passive devices are fabricated on LCP laminate based substrates. In order to improve the surface roughness of the LCP substrates, the surface is pre-treated by means of oxygen plasma etching. In addition, a thin adhesion layer of Titanium (Ti) is applied prior to the sputter deposition of Cu to improve the adhesion between the Cu and the LCP material. The comparison of simulated and measured results of both developed filters and antennas is also shown and discussed. The fact that both UWB bandpass filters and UWB antennas show good results, verifies that LCP materials are potentially promising substrate material for microwave/mm-wave applications.