Stresses in Pt/Pb(Zr,Ti)O3/Pt thin-film stacks for integrated ferroelectric capacitors

Abstract

A study of the stresses in a ferroelectric capacitor stack deposited on an oxidized silicon substrate is presented. The capacitor stack was prepared with sputtered Pt bottom and top electrodes and a ferroelectric film of composition PbZrxTi1−xO3 (PZT) with x≊0.5 which was deposited using a modified sol-gel technique. The stresses were determined by the changes in the radius of curvature of the wafer following the deposition steps, during and after annealing treatments, and after etching steps in which the top electrode, the PZT film, and the bottom electrode were successively removed. The largest stress effects are found in the Pt electrodes which are deposited under conditions giving an intrinsic compressive stress. An annealing treatment exceeding 500 °C changed the stress of the bottom electrode from ≊−750 MPa (compressive) to a large tensile stress (≊1 GPa). This stress is largely thermal and is caused by the differences in thermal-expansion coefficients of the Pt film and the Si substrate. The stress of the PZT film is numerically relatively small (below ≊200 Mpa) and it is found to be of both thermal and intrinsic origin. The deposition and annealing of the top electrode has a profound influence on the stress of the PZT film as well as on the electrical properties. The stress behavior of the as-deposited PZT film shows a poling direction mainly in the plane of the substrate. An annealing of the complete capacitor stack changes the poling direction of the ferroelectric film to be perpendicular to the substrate. This explains the observed electrical switching properties of as-prepared as well as annealed ferroelectric capacitors.

Preferential grain growth and improved fatigue endurance in Sr substituted PZT thin films on Pt(111)/TiOx/SiO2/Si substrates

“…It is evident from the figure that films crystallized in perovskite structure with no distortion of the unit cell corresponding to the tetragonal phase. Similar observation has been reported on PZT film near the MPB [13]. In the case of thin films, broadening of the diffraction peaks, mainly due to the small grain size leading to overlapping of the (0 0 l) (h 0 0) peaks and hence it is difficult to index the individual peaks [14].…”

Development of residual stress in sol-gel derived Pb(Zr,Ti)O3 films: An experimental study

“…However, in their work PZT was deposited on silicon rather than on platinum. Spierings et al 19 reported on the development of residual stress in the sol-gel deposited PZT 50/ 50 films and argued that the nature of stress in PZT is due to thermal expansion mismatch between PZT and the bottom ͑or the top͒ electrode which could be explained in more detail if the anisotropic thermal expansion of PZT is considered. The stress relaxation mechanism of the domain formation at the phase transition was first reported by Roitburd,20 which was further developed by Speck and Pompe and co-workers 21,22 and Pertsev et al 23 for epitaxial thin films by calculating the thermodynamically stable domain configuration dependent on the thermal strain difference between the film and the substrate.…”

Measurement of residual stress of PZT thin film on Si(100) by synchrotron X-ray rocking curve technique

“…Thermal expansion coefficient for PZT gradually increases from 2.5 × 10 −6 at 25 • C to 4.1 × 10 −6 at 600 • C [18] while that of silicon substrate is 3.5 × 10 −6 [13]. During cooling from deposition temperature to room temperature, PZT film undergoes phase transformation from cubic phase to tetragonal phase at Curie point, resulting in transformation stress (σ tr ).…”

“…Another group of methods calculates film residual stresses indirectly from the curvature of the sample surface or substrate due to balance/equilibrium between the forces and moments in the films and the substrate. The latter includes laser scanning technique [17,18], profilometry [19], double crystal diffraction topography (DCDT) [20,21], and optical interferometry [22]. X-ray diffraction sin 2 ψ method is well established and widely used in the measurement of residual stress of the thin films.…”