Variable Temperature Broadband Microwave and Millimeter-Wave Characterization of Electrochromic (WO3/LiNbO3/NiO) Thin Films

Abstract

This paper reports for the first time, the dielectric characterization of a new, electronically tuneable electrochromic (EC) thin-film material over the frequency range 1–67 GHz, at temperatures of 7 °C, 23 °C, and 50 °C. Test cells composed of a microstrip line terminated with coplanar waveguide (CPW) transitions were fabricated to facilitate on-wafer RF measurements and the application of different bias voltages using a standard CPW probe station. A precise curve-fitting technique based on full-wave simulations was used to extract the permittivity and loss tangent values of the material. The validity of the dielectric extraction technique was first demonstrated by employing a known material, silicon dioxide (SiO2). It is shown that the dielectric tunability of the EC material varies between 11.3% (1 GHz) and 7.5% (67 GHz) at 23 °C, and the measured loss tangent varies between 0.012 (OFF, 0 V, state) and 0.025 (ON, 6 V, state). Above room temperature, the devices exhibit higher values of dynamic tunability and a small increase in insertion loss. The results obtained for this first generation of tuneable EC material are encouraging, and many of the dielectric properties are shown to compare favourably with other, more mature bulk tuneable media, such as liquid crystals.