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Effects of H2 Sintering and Pt Upper Electrode on Metallic Bi Content in Sr0.9Bi2.1Ta2O9 Thin Films for Ferroelectric Memories Prepared by Sol-Gel Method

The effects of Pt electrode and H2 sintering on the metallic Bi content of Sr0.9Bi2.1Ta2O9 (SBT) thin films for ferroelectric memories were studied using X-ray photoelectron spectroscopy (XPS). Oxidic Bi in SBT films is reduced to metallic Bi by H2 sintering. The degree of reduction depends on the structure of the SBT film. The SBT film with a fluorite structure is more difficult to reduce than that with a Bi-layered structure. Pt upper electrode formation also leads to an increase of metallic Bi content. Both H2 sintering and the Pt upper electrode work synergistically. In the combination of the pure Pt upper electrode and H2 sintering, over 80% of Bi on the SBT film surface was metallic. Pt films of 5 nm thickness on the SBT film recrystallized during the heat treatment and were broken up into particles. Pt electrode coverage was about 20% after the 2nd annealing at 800°C for 30 min in O2 atmosphere. Electrical properties were significantly affected by the presence of metallic Bi; in particular, SBT films with higher metallic Bi content showed higher leakage current density. The metallic Bi content must therefore be suppressed to obtain SBT films with low leakage current density.