Spin-cast P(VDF-TrFE) films for high performance medical ultrasound transducers

Ferroelectric polymers are used in many special medical and industrial ultrasonic transducer applications. In most of these applications a free-standing metallized (i.e., patterned) film is adhered to a substrate material which is either reflective (i.e., high acoustic impedance) or acoustically matched in impedance but highly lossy. The adhesive layer can have adverse effects on ultrasonic transducer performance, especially at high frequencies. This work focussed on the direct formation of high performance ferroelectric copolymer films onto substrate materials suitable for fabricating medical ultrasound transducers. P(VDF-TrFE) copolymer resin was dissolved into methylethyl-ketone and subsequently spin-coated onto various ultrasound transducer substrate materials. After deposition, the 6-25 μm thick films were cured, crystallized and poled on the substrates at fields of 100-120 V/μm. Several AC and DC poling techniques were used. The films were then characterized for their high frequency properties of interest for medical ultrasound, including their clamped dielectric properties, electromechanical coupling coefficient (k _t), and mechanical quality factor (Q_m). The results yielded k_t values as high as 0.30 and Q_m values in excess of 24. Ultrasonic tests confirmed that high performance broadband ultrasound transducers can be fabricated with ferroelectric polymers without the need for adhesive layers