Low loss tungsten-based electrode technology for microwave frequency BST varactors

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/SiO₂/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.