All-Printed Flexible Microwave Varactors and Phase Shifters Based on a Tunable BST/Polymer

Abstract

This paper presents all-printed varactors and phase shifters using direct-ink writing methodologies on flexible organic films. The key enabler is a novel ferroelectric nanoink that allows printing high dielectric constant, low loss, and electrostatically tunable dielectrics at extremely low temperatures. The dielectric is made by suspending nanoparticles of barium strontium titanate (BST) in thermoplastic cyclic olefin copolymer (COC) to create a multiphase BST/COC nanocomposite. Unlike conventional ferroelectric ceramics, this ferroelectric dielectric requires no sintering and can be printed on any substrate. After printing, it is cured at temperatures below 200 °C. Careful stoichiometry and particle analysis of BST powders resulted in identifying conditions for achieving electrostatic tunability. A high relative permittivity of ${ \varepsilon _\text{r} = 38}$ and a very low dielectric loss of ${\tan \delta = 0.002}$ at $f= 10$ GHz were measured for the printed sinterless dielectric. The ink allows printing voltage-variable capacitors with a capacitance tunability up to 10% at microwave frequencies. The tunable capacitors were utilized in a left-handed transmission line design to create all-printed tunable phase shifters with up to a maximum phase shift of $\Delta \phi = {97}$ ° and a maximum figure of merit of 29°/dB at $f= 10$ GHz.