Effects of post-annealing on dielectric properties of (Ba,Sr)TiO₃ thin films prepared by liquid source cvd

Abstract

(Ba,Sr)TiO₃ [BST] is a material suitable for capacitors in 1Gbit DRAMs and beyond. To apply BST thin films in DRAMs, the conformal step coverage is required along with low leakage current and high dielectric constant. We have already reported that the BST films deposited by liquid source CVD method using Ba(DPM)₂, Sr(DPM)₂ and TiO(DPM)₂ have conformal coverage of > 80% in a trench of aspect ratio ≈ 1.^[1] In this paper, we present improvements in the dielectric constant of BST thin films by post-annealing. The BST films 30-40 nm in thickness were deposited on Pt/SiO₂/Si substrates at a temperature of 420°C. After deposition of upper platinum electrodes and passivating SiO₂ films on BST, samples were annealed for 30 min in N₂ or O₂ atmosphere at temperatures of 600–800°C. The other samples without passivating films were also annealed under the same conditions. As the annealing temperature was increased, the dielectric constant ε of BST films increased for both samples. Regarding the leakage current after post-annealing, little increase was observed for samples with passivating SiO₂ films, although the leakage greatly increased for those without passivating films because of roughening of the interface between the upper electrodes and BST films. Thus, through post-annealing with passivating films, an equivalent thickness of 0.38 nm (ε ≈ 300) and a leakage current of about 2 × 10^×8A/cm²(@ ± 2V) were achieved. The relationshp between dielectric properties and the crystallinity of post-annealed BST films is further discussed to unravel the mechanism responsible for these improvements.