Design and parameter-extraction based small-signal modeling of a novel center-anchor MEMS series switch

A novel center-anchor MEMS series switch is presented in this paper. A signal-line gap of 270 um, which is two or more times broader than that of a conventional MEMS switch, is achieved, constituting a substantial improvement of the isolation characteristic. This is attributed to the formation of a broad gap by the centeranchor, which sufficiently limits off-state coupling. The measured isolation is -58 dB to -34 dB at 5-35 GHz, whereas that of insertion loss is -0.28 dB to -0.34 dB. Also, a parameter-extraction based small-signal c model is newly developed for evaluation of the center-anchor MEMS switch. Accurate agreement between the measured and modeled RF performances demonstrates the validity of the proposed model.

I. Introduction

Recently, micro-electro mechanical system (MEMS) series switches have been used extensively as switches or filters in microwave circuits [1], [2]. These switches show much lower insertion loss and higher isolation characteristic up to K-band compared with those of solid-state devices, such as PIN diodes or FETs. For reason of better RF performance, MEMS series switches have been successfully applied to dual-path power amplifiers for wireless communication systems [3] and to 4-bit true time delay (TTD) phase shifters for telecommunication systems [4]. Although the MEMS switches have been effectively employed in several specific applications, most single-pole multi-throw (SPMT) applications of MEMS series switches, such as transceivers (T/R) or multi-band selectors, are potentially limited by leakage signals caused by the lower isolation characteristic of MEMS series switches, which seriously degrade the quality of communication performance due to the unexpected intermodulation [5]. To overcome the bottleneck for RF performance, particularly off-state RF coupling, in order to avoid degradation of the insertion loss, newly designed switches are needed for SPMT applications. Moreover, to evaluate the RF performance for fabricated metal-contact MEMS switches, a small-signal model is required. However, the conventional models cannot provide proper equivalent circuits to predict exact RF performance as they are non-physically based models. The conventional, two-port series-impedance models [1], [4] are obtained by a numerical approach method using an electromagnetic simulator. As these models do not consider the fringing capacitances to take place practically at the front end of each broken signal line, the conventional models for on-state modeling suffer from inaccuracy. As such, a physically based small-signal model must be used to exactly predict the isolation loss and the insertion loss. Fig. 1.

Photograph of the ca mems series switch in (a) a top view and schematic cross section views (b) in a-a’ and (c) in b-b‘.