12F-3 A Pitch-Catch UHF Piezoelectric Transformer Fabricated with Thin Film PZT Transducers

A novel high isolation piezoelectric transformer operating in the UHF band is presented. The acoustically coupled transformer (ACT) is composed of thin film PZT transducers on both sides of an insulating substrate. The input transducer launches an acoustic wave through the insulating substrate to the output transducer in a pitch/catch mode to realize a 1:1 turns ratio. Appropriate series connection of multiple input transducers and parallel connection of the corresponding output transducers results in a transformer with an N:1 turns ratio. The ACT is fabricated using standard thin film and wafer scale fabrication techniques on a 150 mm diameter alumina wafer. The PZT transducers are deposited on Pt electrodes on both sides of the substrate by a sol-gel method. A backside photo-aligner is used to assure alignment of front and back side devices. Thousands of transformers can be made at once, thus eliminating costly one-off manufacturing. A novel differential electrode structure is used to apply the electric field to the PZT transducers without the need to etch the PZT film to reach the bottom electrode. Scattering (S) parameters of the ACT devices were obtained experimentally with a novel double-sided probe station that used air co-planar microprobes and a differential network analyzer. ACT transformers of 1:1 voltage ratio, with high DC isolation, and operating in the 20 to 200 MHz range are realized. The transformers exhibit a series of low insertion loss resonant peaks, spaced ~12 MHz apart, with |S₂₁| as low as -1.2 dB when conjugate matched at the input and output ports. A temperature coefficient of frequency of -49 ppm/degC is measured. Power densities in excess of 400 Watts/cm³ are achieved. Dispersion diagrams for the allowed vibration modes are presented.