Sputtered Tantalum as a Structural Material for Surface Micromachined RF Switches

Abstract

Micro-electro-mechanical systems (MEMS) are batch fabricated miniature systems with both electrical and non-electrical (e.g. mechanical) functionalities. Examples of such MEMS are miniature mechanical switches, acceleration sensors, gyroscopes etc… RF-MEMS devices are an emerging class of MEMS devices used for RF signal processing in telecommunication applications. For example, metal surface micro-machined switches are a promising alternative for current solid state switches or mechanical reed relays due to their potential for miniaturization, integration and improved performance. RF-MEMS switches normally consist of a metal bridge or cantilever electrode, which can be pulled in towards a bottom electrode by electrostatic forces [1]. The bridge material is often chosen to be Al or an Al-alloy because of its low resistivity [2]. During each switch cycle the metal top electrode goes through a mechanical stress cycle. These successive stress cycles might lead to reliability problems such as fatigue and/or creep. This is certainly the case for Al, which is known to have a low creep resistance [3]. Several alternatives have been investigated as alloying Al with other materials [4], or replacing Al with metals having better mechanical properties, specifically low creep, such as Tungsten (W) or Tantalum (Ta). Especially high melting point materials are interesting alternatives as the creep resistance is expected to increase with the melting point [5]. If we consider surface micromachined RF switches, the pull in voltage and sheet resistance are two important parameters that should be taken into consideration while selecting the switch material. In general, the pull in voltage depends on mean stress, lateral dimensions, layer thickness and Young's modulus. In case of stress free films, a material having a low Young's modulus is preferred for low pull-in voltage. As W is known to have high intrinsic stress [6], and a higher Young's modulus compared to Ta [7], it is instructive to consider Ta as a structural material for surface micromachining applications.