Dielectric properties and microstructure of thin BST films

(Ba,Sr)TiO₃ (BST) has emerged as the preferred high permittivity thin film material for future generations of high-density DRAM products. Liquid delivery metalorganic chemical vapor deposition (MOCVD) has been used to deposit thin films of BST on Pt/SiO₂/Si in a systematic study of thickness and composition effects on microstructure, film orientation and electrical properties. We have found that smooth, thin films (30 nm) with storage densities > 80 fF/μm², low leakage currents (< 10^-7 A/cm²), and low dielectric losses (tan δ ∼ 0.002) can be deposited with high reproducibility when composition is maintained between 1-2 at% excess Ti. Dielectric loss is strongly correlated to roughness, both of which generally increased with increasing thickness. For compositions closer to stoichiometry, larger grains formed at Pt grain boundaries grew faster than the surrounding grains, resulting in higher variability in dielectric loss.