Stress and phase transformations in PZT thin films.

摘要

Thin films of ferroelectric materials such as lead zirconate titanate, Pb(Zr_xTi_1−x)O₃ or PZT, have a variety of applications in sensors, actuators and non-volatile memory devices. While bulk ferroelectric materials have been investigated extensively in the past, the differences that exist between thin film and bulk PZT can significantly affect the structural and electrical properties of PZT capacitors.; We have investigated the effects of film stress and thickness on the structural and electrical properties of nominally tetragonal Pb(Zr_0.35 Ti_0.65)O3 films ranging in thickness from 700Å to 4000Å grown by metalorganic chemical vapor deposition. Using high-resolution x-ray diffraction, electron backscatter diffraction and electrical measurements, we have observed that as-deposited PZT films of 1000Å and less are rhombohedral, while the thicker films are partially tetragonal and partially rhombohedral. X-ray depth profiling was used to show that the thicker as-deposited PZT films have a layered structure, with tetragonal PZT near the surface and rhombohedral PZT near the substrate interface. The origin of the rhombohedral phase in the nominally tetragonal PZT can be explained by considering the effect of substrate constraint on PZT grains that contain a single ferroelastic domain. Wafer curvature and x-ray diffraction stress measurements were used to show that the observed structure of PZT films is consistent with the theoretically predicted stress-induced phase transformation from the equilibrium tetragonal into the rhombohedral phase. The observed thickness-dependent structural evolution affects the electrical properties of the PZT films. Annealing the films changes the relative volume fractions of the rhombohedral and tetragonal phases and the ferroelectric and dielectric properties of the PZT capacitors.; We have also used a wafer bending method to investigate the effect of applied mechanical strain on the ferroelectric and dielectric properties of PZT capacitors. It was observed that even small biaxial strains can affect the electrical properties of PZT, significantly decreasing the remanent polarization and increasing the dielectric constant of PZT capacitors. These observations are consistent with the accommodation of imposed strain by reversible 90° domain wall motion that changes the volume fraction of domains that switch during electrical testing.