Design of ferroelectric phase shifters for minimum performance variation over temperature

This paper investigates design techniques for continuously variable ferroelectric phase shifters to minimize their performance variations over temperature. Various compositions of barium strontium titanate (BST) thin films on sapphire substrates were characterized in the temperature range of 0°C-80°C to assess their change in capacitance and tenability. It was found that a minimum capacitance variation (C_max/C_min) of around 1.17 could be obtained using a Ba/Sr ratio of 1.2 and (Ba+Sr)/Ti ratio of 0.8. Based on this composition, all-pass network phase shifters were designed using 2 μm- and 4 μm-spaced interdigital capacitors (IDCs). In the case of the phase shifter with 2 μm-spaced IDC, a phase shift of more than 100° was obtained over a 40 V bias range at 2.4 GHz. The loss figure-of-merit (FOM) of the phase shifter was held to 62.5°/dB ± 2.5°/dB from 10°C to 75°C. The variation of a phase shift was held to only 4° in the temperature range of 10°C-50°C.