Microstructure and Chemistry of Dielectric Thin Films

To achieve acceptable leakage currents and dielectric lifetimes for DRAM applications, Ba_1-xSr_xTiO₃ (BST) films must be deposited with an excess of Ti. However, the dielectric constant decreases with deviation from ideal stoichiometry; e.g., the dielectric constant decreases by 50% when the Ti content is increased from 51.0% to 53.5%. The objective of this project was to identify potential microstructural mechanisms for the accommodation of excess Ti, which is critical to optimizing the material performance and deposition process.

To address this issue, high-spatial resolution electron energy loss spectroscopy (EELS) and high resolution electron microscopy studies were conducted on BST-DRAM films with Ti contents ranging from 50.7% to 53.4%Ti. The films, obtained from industry, were deposited on Pt/SiO₂ /Si substrates by MOCVD. Examination of the cross-sectional specimens showed that the BST film was composed of columnar grains that extended through the entire film.

Fig. 1 Phase contrast image of the BST (53.5% Ti)/Pt interface.

No second phase or amorphous interlayer was observed at the BST/Pt interface (Fig. 1), but some of the BST grain boundaries contained small "pockets", 2 nm to 3 nm in size, with amorphous-like contrast (light areas in Fig. 2).

Fig. 2 Structural image of the planar view specimen (53.4% Ti).

Maps of the Ti-L_2,3 and the Ba-M_4,5 absorption edge intensity distributions were obtained using EELS spectrum-imaging; these data were quantified using single crystal BaTiO₃ as a standard. The measurements showed that the grain boundaries in all of the films had a higher Ti/Ba ratio than the grain interiors (Fig. 3).

Fig. 3 Maps of Ti-L_2,3 and Ba-M_4,5 absorption edge intensities and the calculated map of the Ti/Ba ratio for the 50.7% Ti film. Lighter contrast in the ratio map corresponds to regions with a higher Ti/Ba ratio.

The results suggest that deviations of the Ti/(Ba+Sr) ratio from the stoichiometric value of unity are accommodated by the creation of Ba/Sr vacancies which segregate to the grain boundary regions. Thus, the BST films with Ti contents ranging from 50.7% to 53.4% consist of at least two phases with distinct chemical composition: grain interiors and grain boundaries. In addition, the films with % Ti > 52% contained an amorphous Ti-rich phase at some of grain boundaries and multiple grain junctions; the amount of this phase increases with increasing overall Ti content. Since the grain size was found to be independent of the average composition, the grain boundaries alone cannot account for the strong compositional dependence of dielectric constant. The analyses further indicate that the amorphous phase can only partially account for the significant drop in dielectric permittivity accompanying increases in the Ti/(Ba+Sr) ratio.

Some of the work presented here is also available from our publications page.