Phase Sequences in Processing

Abstract

PZT and SBT exist in several different phases. When PZT films are deposited under normal conditions the phase that is formed has the pyrochlore structure. These pyrochlores are generally ferroelectric but only at cryogenic temperatures; thus, they are not useful for room temperature ferroelectric memory devices. The pyrochlores deposited by sputtering, sol—gel spin-on, or other techniques are subsequently subjected to an annealing cycle, either in a furnace (typically an hour at 650°C) or via RTA (Rapid Thermal Annealing), typically for 90 seconds to a maximum temperature of about 800°C. These heat treatments result in 100% of the material being converted to the ferroelectric perovskite phase. Usually this heating process is carried out in an oxygen atmosphere to prevent oxygen loss from the film. The lowest temperature at which 100% of the film has been successfully converted to perovskite is 450°C [334]. Recently it has been suggested [334] that the pyrochlore phase is stabilized by Pb⁴⁺ ions in PZT, and efforts are underway to reduce this Pb⁴⁺ concentration to see if even lower temperature conversion to the perovskite phase is possible.