Photodefinable Metal Oxide Dielectrics: A Novel Method for Fabricating Low Cost RF Capacitive MEMS Switches
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Photodefinable Metal Oxide Dielectrics: A Novel Method for Fabricating Low Cost RF Capacitive MEMS Switches Guoan Wang2, Augustin Jeyakumar1, John Papapolymerou2, and Clifford L. Henderson1* 1 School of Chemical & Biomolecular Engineering, Georgia Institute of Technology, 311 Ferst Drive NW, Atlanta, GA 30332-0100 2 School of Electrical and Computer Engineering, Georgia Institute of Technology, 777 Atlantic Drive NW, Atlanta, GA 30332-0250 * Corresponding Author: E-mail: [email protected], Phone: (404)-385-0525 ABSTRACT In this paper, a novel approach for fabricating low cost capacitive RF MEMS switches using directly photodefinable high dielectric constant metal oxides has been developed. In this approach, a radiation sensitive metal-organic precursor is deposited via spin coating and converted patternwise to a high dielectric constant metal oxide via ultraviolet exposure. The feasibility of this approach is demonstrated by fabricating bridge-type and cantilever-type switches with a nitride/metal-oxide/nitride dielectric film stack. These switches exhibited significantly higher isolation and load capacitances as compared to comparable switches fabricated using a simple silicon nitride dielectric.
1. INTRODUCTION Low-cost MEMS switches are prime candidates to replace the conventional GaAs FET and p-i-n diode switches in RF and microwave communication systems, mainly due to their low insertion loss, good isolation, linear characteristic and low power consumption. Various designs of capacitive RF micromechanical switches made out of nickel [1], aluminum [2,3], gold [4], and copper [5] have been so far reported in literature, with a variety of intended applications such as phase shifters, reconfigurable filters, tuners and other planar circuits. The structure of these switches consists of a lower electrode, a very thin dielectric layer, and a moveable membrane. Several studies have shown the importance of the dielectric layer in the switch performance (Cmax/Cmin ratio, isolation) and reliability [6]. In most MEMS switches reported so far, this dielectric layer is typically silicon nitride deposited with PECVD or HDICP CVD techniques [7]. A switch that uses BST as the dielectric layer has also been reported [8]. Taking into account the higher cost associated with using CVD and sputtering techniques for fabrication, there is a need for lower cost switches based on inexpensive dielectric layer fabrication techniques, while maintaining flexibility in the choice of the dielectric parameters (dielectric constant and loss). This paper presents capacitive RF MEMS switches made with a very low fabrication cost oxide dielectric layer, whose parameters can be easily controlled during fabrication to achieve the desired values. In this process, a photosensitive metal-organic precursor solution is cast onto the substrate (typically by spin coating) to form a thin, solid, amorphous precursor film. Upon UV exposure, the organic ligands of the precursor molecules are cleaved, resulting in the formation of an amor
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