Journal of Electroceramics, 13, 215–222, 2004 C  2004 Kluwer Academic Publishers. Manufactured in The Netherlands. Polar Ceramics in RF-MEMS and Microwave Reconfigurable Electronics: A Brief Review on Recent Issues N. SETTER, V. SHERMAN, K. ASTAFIEV & A.K. TAGANTSEV Ceramics Laboratory, EPFL Swiss Federal Institute of Technology, 1015 Lausanne, Switzerland Submitted January 24, 2003; Revised March 4, 2004; Accepted March 4, 2004 Abstract. Properties and fabrication status of microdevices for microwaves based on polar ceramics are reviewed. We discuss bulk acoustic wave devices with AlN films, rf-MEMS capacitive switches with high permittivity materi- als, and tunable ferroelectrics. The relevant properties of ferroelectrics for microwave applications are summarized with emphasis on composites and thin films. Keywords: ferroelectric, tunable, microwave, AlN, BST, piezoelectric, bulk acoustic waves, rf switch, rf-MEMS 1. Introduction With the intensification of use of electronic com- munications, it becomes essential to produce high frequencies electronic systems that are miniaturized, reliable, and of low cost yet high performance. This is particularly true for integration of passive devices since passives occupy today nearly 60% of the total area of handheld devices. Due to their large size, and sometimes the need of special materials (e.g. LiNbO 3 crystals for surface acoustic wave devices) many microwave components such as filters and antennas are often placed outside the chip package, requiring inefficient connections, and introducing parasitic inductance into the system—miniaturization and integration of these components onto one substrate can be highly advantageous for cost and size reduction [1]. Miniaturized tunable capacitors open interesting perspectives for new active rf components: Minia- turized tunable filters, resonators, phase-shifters, antennas, etc. can potentially be manufactured with the desirable characteristics of very high Q (quality factor), narrow bandwidth, low power consumption, low insertion loss, high isolation, and high speed, replacing bulky, multi-component systems. The field of rf-MEMS (we refer to the wide definition of MEMS as microsystems not necessarily including moving parts) is being intensively developed for this end. The challenge in comparison of other MEMS containing devices is the making of high frequency circuitry that is compatible with MEMS technology. An area that received ample attention recently is that of rf-switches. These are either resistive or capacitive switches in which contacts are formed by actuation of a cantilever (usually a metallic cantilever actuated by electrostatic force) either with another metal (resistive switch) or with an insulator which is placed on top of a conductor (capacitive switch). An array of such elements, in which each element is individually addressed is often used. In parallel to the current development trends in IC technology, also MEMS developers realize that limiting the technology to that of standard materials such as Si, silicon oxide and silicon nitride does not necessarily provide the most efficient (performance to cost) and competitive solutions to miniaturization and integration components. Functional materials: piezo- electrics, ferroelectrics, magnetic, etc. are therefore being introduced. We limit this paper to polar ceramics. Three groups of rf-MEMS components make use of polar ceramics: (1) acousto-electric components based on bulk acoustic waves [BAW] that control the trans- mission characteristics of a circuit utilizing the piezo- electric effect, (2) capacitive switches in which the di- electric layer is replaced by a high dielectric constant material, and (3) tunable capacitors in which tunability, namely the electric field dependence of the permittivity of ferroelectric materials is used to modify actively the capacitance of the circuit. The paper summarizes the