Journal of Electroceramics, 14, 75–81, 2005 C  2005 Springer Science + Business Media, Inc. Manufactured in The Netherlands. Low Loss Tungsten-Based Electrode Technology for Microwave Frequency BST Varactors J.-P. MARIA, 1, ∗ B.A. BOYETTE, 1 A.I. KINGON, 1 C. RAGAGLIA 2 & G. STAUF 2 1 Department of Materials Science and Engineering, North Carolina State University, Raleigh, 27695-7920, NC, USA 2 Advanced Technology Materials, Inc., Danbury, CT 06810, USA Submitted January 16, 2004; Revised June 16, 2004; Accepted June 18, 2004 Abstract. Composite metallization stacks consisting of 1.5 µm W layers and 0.1 µm Ir layers have been developed for low series resistance electrode applications. These hybrid metallization layers are compatible with barium strontium titanate deposition and standard patterning/etching procedures. The multilayer stacks were prepared for use as bottom electrodes in microwave varactors based upon the electrically tunable dielectric barium strontium titanate. These low resistivity layers are critical for realizing low insertion loss devices, and overcome difficulties associated with delamination or hillocking in traditional noble metal layers. Controlling the oxygen content during BST deposition was necessary to achieve high-quality BST and a stable electrode stack. Using these metallization stacks and optimized deposition conditions, BST films with tan δ values below 0.007 and tunabilities of 2:1 were realized. These properties are comparable to those measured on typical Pt/BST/Pt/SiO 2 /Si capacitors. Finally, a combination of reactive ion and wet chemical etching were used to demonstrate that the thick hybrid metallization layers are patternable using routine and manufacturable process methods. Keywords: microwave, BST, electrode, ferroelectric, metallization 1. Introduction Replacing semiconductor varactors with electrically tunable dielectrics is of technological interest given the potential improvements in high frequency loss, device reliability, and miniaturization [1]. The elec- trically tunable dielectric of greatest current interest is (Ba,Sr)TiO 3 with a composition and thickness ad- justed such that the ferroelectric transition temperature is close to, but below the predicted operating ambient [2]. Producing metal-insulator-metal capacitors with BST dielectric layers has been routinely demonstrated, with loss tangent values <1% and tunabilites equal to or greater than 2:1. Though these loss tangents will yield dielectric Q values approaching 1000, total de- vice Qs in this range have not been reported. In part, this results from the lack of a suitable electrode tech- nology offering low dc resistance. The quality factor of ∗ To whom all correspondence should be addressed. E-mail: jpmaria@ncsu.edu a tunable capacitor can be expressed in the following manner: 1 Q total = 1 Q BST + 1 Q metal = tan δ + ωC P R S where the contributions from the dielectric loss, capaci- tance, frequency, and series resistance can be separated. This behavior is graphically represented in Fig. 1. Note in Fig. 1 that for a 1 GHz, 1 pF capacitor, if a Q fac- tor of 100 is desired, the metallization cannot supply more than 5  of total resistance. For Pt electrodes, a 50 µm wide line prepared with the common 100 nm thickness with provide this resistance value in a length of only 250 µm. This example represents only a first order approximation of a complex system, however, it does accurately depict one limiting case where met- allization loss provides the predominant influence [3]. To overcome this obvious limitation, lower resistivity structures are necessary. The simplest way to reduce the metal series resistance is through increasing the elec- trode thickness. Unfortunately, however, increasing the