1 Introduction

The end of the 20 ^th century experienced major evolutions in the radio-frequency (RF) technologies, [De Los Santos 1, Itoh, Katehi 1]. Exponentially growing interest in mobile communications and networking of information have brought new challenges, with particular emphasis on more affordable and integrable technologies able to provide superior RF functionalities per unit of volume. Immediate impact has been observed on the research and development of silicon-based RF integrated circuits (ICs) with special emphasis on sub-micron silicon CMOS, SOI and SiGe hetero-junction bipolar transistors. New technologies have also emerged, stimulated by the prospect of enormous markets. One successful example is micro-electromechanical-systems (MEMS) technology. Even if first developments in RF MEMS originated from the requirements of airborne systems [Brown], nowadays the telecom mass market [De Los Santos, Nguyen 1, Tilmans] is the main driver of this field. Even though first MEMS devices were demonstrated in the late 1960's [Petersen], major developments for applications had had to wait for more than 30 years because of non-adapted technologies.

Generally, MEMS are considered as the merger of the IC world with the micronscale mechanical world. Conventional IC fabrication techniques stand along with more exotic processes, in order to provide MEMS architectures. MEMS devices are extremely attractive for RF ICs (especially wireless) because of their major gains in terms of: (i) device and system miniaturization (resulting in lower costs), (ii) integration (same planar technology as...