BaxSr1−xTiO3Thin Film Ferroelectric-Coupled Microstripline Phase Shifters with Reduced Device Hysteresis

Abstract

In this work, we present experimental results on the performance of coupled microstripline phase shifters (CMPS) fabricated using BaxSr1−xTiO3 (BST) ferroelectric thin films. The CMPS were fabricated using commercially available pulsed laser deposition BST films with Ba:Sr ratios of 30:70 and 20:80. Microwave characterization of these CMPS was performed at upper Ku-band frequencies, particularly at frequencies near 16 and 18 GHz. The quality of performance of the CMPS was studied based on their relative phase shift (Δϕ=ϕnV−ϕ0V, where n=0–400 V DC) and insertion loss within the DC bias range of 0–400 V (i.e., E-field ranges within 0–53 V/μm). The performance of the CMPS was tested as a function of temperature to investigate their operation in the paraelectric as well as in the ferroelectric state (i.e., above and below the Curie temperature, respectively). It was observed that for the CMPS fabricated with the BST films, with Ba:Sr ratio of 30:70, the performance was very good with Δϕ up to 400° and nominal insertion losses of ∼3 dB. In addition, these devices were hysteresis-free in the paraelectric state and only showed Δϕ hysteresis while performing in the ferroelectric state. In contrast, while the CMPS made with Ba:Sr ratios of 20:80 exhibited very good values of Δϕ, they exhibited a mild hysteresis both in the paraelectric and ferroelectric states. X-ray diffraction studies indicate that the 30:70 films exhibit almost a 1:1 ratio between the in-plane and out-of-plane lattice parameters, suggesting that their cubic structure creates strain-free films conducive to CMPS devices with reduced hysteresis in the paraelectric state. Elimination of Δϕ hysteresis is essential because many practical microwave applications such as voltage-controlled oscillators and beam steerable devices, particularly electronically steerable phased array antennas, rely on accurate phase shift versus tuning voltage profiles required for reliable device operation. Accordingly, the optimization of the interplay between film microstructure, Ba content, and dielectric constant is critical for reliable CMPS devices.