Recent changes in legislation restricting lead content in waste streams produced during electronics manufacturing have inspired a strong push to develop alternative lead-free materials systems for piezoelectric applications. One lead-free piezoelectric material system, (100-X)Bi₀.₅Na₀.₅TiO₃-XBi₀.₅K₀.₅TiO₃ (BNKT), has shown promising properties in bulk research, however, difficulties have arisen in the development of marketable thin film BNKT. The first part of this study focuses on process optimization for a thin film BNKT chemical solution deposition process using a rapid thermal annealer (RTA). Various process conditions were evaluated based on their impact on key film properties including crystallinity, surface morphology, cross sectional microstructure, dielectric properties, ferroelectric response and piezoelectric response. The process conditions evaluated include: anneal temperature, anneal time, ramp rate, oxygen flow, excess cation content, and pyrolysis temperature.
The majority of BNKT research has been done near the observed MPB composition of 80Bi₀.₅Na₀.₅TiO₃-20Bi₀.₅K₀.₅TiO₃ due to its characteristically large piezoelectric response. The second part of this study evaluates the impact of composition on thin film BNKT properties. Three separate compositions were evaluated: 40BNT-60BKT, 60BNT-40BKT, and 80BNT-20BKT. It was found that dielectric loss is considerably lower in 40BNT-60BKT (tan δ = 0.02-0.05) than at the MPB (tan δ = 0.05-0.1). Piezoelectric response is maximized near the MPB with d33 ranging from 30-40 pm/V at 40BNT-60BKT, 50-60pm/V at 60BNT-40BKT, and 60-90pm/V at the MPB. Composition experiments were performed using a box furnace anneal method in addition to the optimized RTA process. This was done in order to illustrate the impact of process optimization on realized film properties. While little impact was shown on piezoelectric response between the two compositions considerable differences are observed in cross sectional microstructure, film crystallinity, and dielectric loss. Films produced using a box furnace instead of an RTA were considerably less dense, showed inconsistent dielectric loss, and for compositions off the MPB considerably lower 100 to 110 x-ray diffraction peak intensity ratios compared to those made in the RTA.