Effects of H₂ sintering and Pt upper electrode on metallic Bi content in Sr_0.9Bi_2.1Ta₂O₉ thin films for ferroelectric memories prepared by sol-gel method

The effects of Pt electrode and H₂ sintering on the metallic Bi content of Sr_0.9Bi_2.1Ta₂O₉ (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.