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 = 21µC/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 K_xNa_1-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. %).