Phase segregation in KxNa1-xNbO3 films grown by large-scale confocal RF sputtering

As of now, KNN is the most viable alternative to PZT. Sputtering deposition is very suitable for large-scale production of thin films, but some challenges need to be overcome. We deposited KNN films on 8” wafers by using an EVATEC's Clusterline-200E sputtering machine. Lower growth pressure and high annealing temperatures promote larger deposition rates, higher crystallinity, and larger c-lattice parameters of the perovskite phase. KNN films thus obtained display a general {001} preferred orientation out of plane with evident appearance of the (001) polar direction. Therefore, ferroelectric response is observed in most films, with a maximum 2Pr = 21 µC/cm2 for films grown at low pressure and annealed at 700 °C under N2+O2 atmosphere. Depending on growth and annealing conditions, however, initially amorphous KNN films crystallize into segregated Na-rich polar perovskite and spurious pyrochlore phases. A Na-rich perovskite ferroelectric phase forms into “volcano-like” islands with composition KxNa1-xNbO3, where x = 0.2–0.3. The stoichiometry aligns with other known non-equimolar KNN morphotropic phase boundaries and is supported by first-principles calculations. As K diffuses away from the islands and escapes from films at a higher rate than Na, the remaining part of the film forms a chemically unstable K-rich pyrochlore phase, of the type K4Nb6O17, that easily degrades during lithographic patterning. We suggest that enhanced KNN films can be produced by RF sputtering not only by counteracting Na and K loss by using alkali-excess targets, but also by employing over-stoichiometric targets with compositions richer in Na (70–80 at%).