A 20 GHz MOD-made BST thin film tunable phase shifter for phase adjustment of digital 360-degree PHEMT phase shifter

We have newly designed and fabricated both Ba_xSr_1-xTiO₃ (BST) ferroelectric thin film tunable phase shifter and pseudomorphic HEMT MMIC digital 360-degree phase shifter. A low loss BST thin film was obtained on MgO substrate by preparation of initial layer by pulsed laser deposition (PLD) and following metal-organic-decomposition (MOD) method. For the interdigital capacitors with finger spacing of 10 μm, dielectric loss was found to be as low as 0.002 to 0.004 when applied surface electric field was from ∓40 to ±40 kV/cm at measuring frequency of 1 MHz, where tunability was about 12%. Moreover, it increases up to about 40% in a Pt/BST/Pt stacked capacitor structure when the applied electric field was from ∓170 to ±170 kV/cm at the same frequency. When applying dc bias voltage of 0 to 60 V to the electrodes of the CPW pattern (width:60 μm, gap:10 μm, length:2.5 mm), a differential phase shift of 18 degree was obtained at 20 GHz with insertion loss of about -2 dB for Au/Cr interconnection. A 3-stage LC-ladder-type phase shifter with variable capacitors of BST film was designed to have a differential phase shift of about 40 degrees at 20 GHz. A fabricated phase shifter shows successfully the shift of 40 degree at 20GHz with bias of 60 V. The HEMT MMIC also shows a digital 360-degree phase shift with 11.25 degree interval, thus the BST phase shifter can be usable for phase adjustment of the MMIC. Finally it is found that the new BST film process is very promising for realizing a micro and millimeter-wave tunable device.