On the Temperature Dependence of the Piezoelectric Response of Prepoled Poly(vinylidene fluoride) Films

There is growing interest in integrating piezoelectric materials with complementary metal-oxide-semiconductor (CMOS) technology to enable expanded applications. A promising material for ultrasound transducer applications is poly(vinylidene fluoride) (PVDF), a piezoelectric polymer. One of the challenges with PVDF is that its piezoelectric properties can deteriorate when exposed to temperatures in excess of 70 °C for extended periods of time during fabrication. Here, we report on the effects of both shortening annealing times and providing this heating nonuniformly, as is characteristic of some processing conditions, on the piezoelectric coefficient (d33) of PVDF films for various thicknesses. In this case, no degradation in the d33 was observed at temperatures below 100 °C for anneal times of under 1 min when this heating is applied through one side of the film, making PVDF compatible with many bonding and photolithographic processing steps required for CMOS integration. More surprisingly, for one-sided heating to temperatures between 90 and 110 °C, we observed a transient enhancement of the d33 by nearly 40% that lasted for several hours after these anneals. We attribute this effect to induced strain in these films.