Barium strontium titanate, $(Ba,Sr)TiO_3$(BST), thin films has attracted great interest due to its high dielectric constant which may realize high capacitance density in ULSI devices such as DRAMs. In the present study, we fabricated stoichiometric $(Ba_0.5 Sr_0.5)TiO_3$ thin films by conventional rf magnetron sputtering onto optimized $RuO_x$-based electrodes ($RuO_2$, Ru and $Pt/RuO_2$), which were deposited by DC reactive sputtering, and then systematically investigated the effects of the bottom electrode and preparation conditions for BST thin films on the electrical properties of BST thin films. The relationship between electrical properties, such as capacitance and leakage current, and microstructures of BST thin films were also studied.
Surface morphology and crystallinity of $RuO_2$ and Ru films for bottom electrode changed with deposition temperature and oxygen content in plasma discharge for $RuO_x$. The $RuO_2$ films deposited at 300℃ and Ar/$O_2$=50/50 have polycrystalline structure and smooth surface. The Ru films deposited at 400℃ have perfectly (002) preferred orientation and dense surface with hillock-free morphology.
Annealing characteristics of Ru and $RuO_2$ thin films were studied in oxygen, argon and vacuum ambient. By annealing in the oxygen ambient, $RuO_2$ thin films were evaporated at 800℃ and Ru films were oxidized above 400℃. Vacuum annealing caused the reduction of ruthenium dioxide to ruthenium. It was also shown that $RuO_2$ bottom electrodes underwent reduction and re-oxidation during the BST deposition procedure. These reactions resulted in the roughening of the surface morphology of BST thin films deposited on $RuO_2$, and hence, high leakage current ($5×10^ {-5} A/㎠$ at 1V). By controlling oxygen partial pressure through oxygen flow heating, it was possible to achieve BST capacitors with a smooth surface and leakage currents of $2 ×10^ {-7} A/㎠$ at 1V, about two orders of magnitude lower than those of BST thin films prepared by vacuum heating.
The thermal stability of $RuO_2$ bottom electrodes with various intermediate layers were investigated to study the applicability of $RuO_2$/diffusion barrier layer for the bottom electrode of integrated ferroelectric capacitor. Sputtered-TiN, CVD-TiN and Ru intermediate layers of about 1000Å thickness were used to prevent oxygen diffusion toward poly-Si substrate. $RuO_2$ deposition at $300℃ did not oxidize the top of sputtered-TiN and Ru buffer layers. $RuO_2$ thin films could not prevent the diffusion of oxygen through itself and oxidation of the underlying TiN layers during oxygen anneals at 650℃, which also caused the outward diffusion of Ti and N. Ru intermediate layer provided a barrier to oxygen diffusion under the same annealing conditions, Which seemed to result from the smaller crystallographic lattice and the less reactivity of Ru thin films than TiN layer.
Thin platinum interlayer of 100 Å thickness was used to reduce the leakage current of BST films deposited on $RuO_2$ bottom electrodes. The leakage current density of the 1000 Å thick BST thin films deposited on 400℃-Pt/$RuO_2$ hybrid bottom electrode was $8.6×10^{-8} A/㎠$ at at 1.5V which was about one order of magnitude lower than leakage current density ($5.9×10^{-7} A/㎠$ at 1.5 V) of BST capacitor with $RuO_2$ films. By controlling the interface band structure through Pt interlayer between BST films and $RuO_2$ bottom electrode, it was possible to achieve the reduction in leakage current of BST capacitors.
The electrical properties of $(Ba,Sr)TiO_3$ thin films with 2-step deposition and various bottom electrodes were investigated to understand the origin of the time-dependent leakage characteristics. BST thin films have interfacial layer of about 70 Å thickness between BST and bottom electrode, which may be considered as oxygen depletion region. In order to modify the oxygen content in the interface, BST films were fabricated by single deposition and 2-step deposition - intermediate layer(100% Ar and 100% $O_2$) and main layer. Such modification made the time-dependent leakage characteristics of BST capacitor changed with various intermediate layers. In the case of 100%-Ar intermediate layer, BST films showed larger exponent value(n) than others. BST capacitors with Pt and Pt/$RuO_2$ hybrid electrode, independent of deposition technique, show time-dependent leakage properties. The decreasing rate of with time depends on deposition method of Pt. On the other hand, dielectric relaxation current was not observed for $RuO_2$ oxide electrode. From the dependence of relaxation current on interfacial layer and electrode material, it is concluded that the characteristics of time-dependentleakage current is dominantly controlled by the density of oxygen vacancy in the interfacial depletion layer and depletion region width.
