Annealing of lead zirconate titanate (65/35) thin films for ultra large scale integration storage dielectric applications: Phase transformation and electrical characteristics

Thin film capacitors of Lead Zirconate Titanate (PZT, 400 nm) of Zr/Ti ratio 65/35, deposited by reactive dc-magnetron sputtering, with low leakage current and high charge storge density(’Qc) for use as capacitor dielectrics in ultra-large scale integration dynamic random access memory (ULSI DRAM) cells have been fabricated and studied. The equivalent SiO2 thickness for the optimized film is 5.3å for the fresh film and 9.1å after 1010 unipolar stress cycles (0 to -3 V). The leakage current density is 1.32 × 10-7A/cm2 for 3 V operation which is equivalent to an effective SiO2 field of 55 MV/ cm. X-ray diffraction analysis reveals that as-deposited films (Tdep. = 200‡ C) contain no detectable perovskite phase. Post-deposition annealing is therefore essential, and critical to the fabrication of high quality capacitors for memory applications. The pyrochlore-to-perovskite phase transformation, the evolution of the microstructure, composition and the presence of different phases in the film have been studied as a function of annealing conditions. There is a purely outward radial growth of the interphase boundary, resulting in the increase in the curved surface area of the cylindrical perovskite aggregates throughout the film thickness with increasing thermal budget. The increase in perovskite phase content with annealing time at a constant annealing temperature indicates that a diffusional phase transformation from the pyrochlore to cubic perovskite phase above the Curie temperature occurs as a first step in the formation of the ferroelectric perovskite phase. The variation of two important dielectric properties, charge storage density and leakage current density is reported as a function of the annealing time and temperature. Furthermore, the variation of the charge storage density due to unipolar dynamic electrical stress is studied. The total area under the large-frequency C-V curve (which is the total reversible polarization) increases under unipolar dynamic stress (0 to -3 V) after 1010 stress cycles. The degradation in charge storage density is found to be primarily due to an increase in remanent polarization caused by the shift in the hysteresis loop as a result of the reduction in the internal bias field under the influence of the unipolar dynamic stress.