Progress in economically viable phase shifters based on thin ferroelectric films

Abstract

A considerable amount of work has been done in the area of frequency and phase agile microwave circuits employing thin ferroelectric films. The best films to date have been Ba_xSr_1-xTiO₃ (BST) grown by pulsed laser ablation. But laser ablation does not seem readily scalable for volume manufacturing, partly because the process appears limited to 5 cm diameter wafers or smaller. Another impediment to commercialization is testing. Testing is cumbersome because the devices must be inserted into a fixture to interface with coaxial launchers. And, high voltage bias tees are not commercially available to safeguard the microwave instrumentation. We have investigated alternate growth methods including MOCLD, CCVD, and sputtering and developed an on-wafer testing approach for screening phase shifters. Some of the results are reported in a companion paper. Here we report results for sputtered PLZT films on LaA1O₃. Coupled microstrip phase shifters at K-band produced a figure of merit of ≈29°/dB. This is within a factor of two of the best laser ablated BST films. The devices also had an uncharacteristically flat frequency response between 12 and 20 GHz. Considering the large lattice mismatch between LaA1O₃ and PLZT, the film crystallinity is very good as determined by XRD. Surprisingly, the films also appeared to be cubic. We also report results for CCVD Ba_0.57Sr_0.43TiO₃ on sapphire. 20 GHz phase shifters yielded a figure-of-merit of 18°/dB. Finally we report the design and performance of a coplanar-to-microstrip transition that permits the phase shifters to be tested on wafer. Devices that pass have the virtual ground probe pads diced off so the phase shifter can be integrated into a phased array. Bias is fed directly through the coplanar probes and isolated from the network analyzer by custom high voltage bias tees.

Key Words:

• Lead zirconate titanate
• Phase shifters
• Barium strontium titanate