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 Pt-base electrodes (Pt/Ta,Ti/$SiO_2$/Si) and then systematically investigated the effects of the Pt electrode and preparation conditions for BST thin films ($O_2$ content in plasma and deposition temperature) 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 Pt films for bottom electrode changed with deposition temperature for Pt. The Pt films deposited at 400℃ by sputtering have perfectly (111) preferred orientation and dense surface with hillock-free morphology.
The crystal orientation and crystallinity of BST films are strongly dependent on the deposition temperature and Pt bottom electrode. BST films deposited on Pt/$SiO_2$/Si below 600℃ have the cubic perovskite structure and show the polycrystalline state. The crystallinity of the films increases with increasing deposition temperature. Films deposited at 600℃ and 650℃ have textured structure with (110) and (100) orientations, respectively. At 650℃, highly (100)-oriented BST films were obtained. However, BST films deposited on Pt electrodes which contain the barrier layer have the polycrystalline structure.
It could be revealed that the crystal structure and electrical properties of high dielectric materials, BST thin films, is strongly dependent on the surface morphology and crystallinity of Pt films for bottom electrode.
Increase of $O_2$ plasma content improve dielectric and insulating properties of the films. An 80nm-BST thin film deposited on Pt/Ta/$SiO_2$/Si with 50% $O_2$ plasma content has a dielectric constant of 380 and a leakage current density of $1.5 \times10^{-7}A/㎠$ at 3V. In the current-voltage curve of BST film deposited with higher $O_2$ plasma content, a wider saturated region of hopping conduction appeares. This indicate that increase of $O_2$ content improve the insulating characteristics of BST films. Qualitatively, it can be said that an increase of $O_2$ content in the film affects the image force at the barrier of conduction thereby decreasing leakage current density and increasing the voltage point to change the conduction mechanism to electron hopping. This phenomena may be advantageous for DRAM devices. With increasing deposition temperature, the crystallinity of the BST films abruptly increased and change of the preferred orientation was observed. The increase of the crystallinity of films improved the dielectric constant. A 100nm BST thin film deposited at 650℃ with 50% $O_2$ plasma content has a dielectric constant of 725 and a leakage current density of $2.3 \times10^{-7}A/㎠$ at 2.5V. In the current-voltage curve of BST film deposited at higher substrate temperature, lower leakage current density in the low bias region and narrower flat region (hopping conduction region) were obtained. This suggests that increasing the crystallinity of the BST films induces the leakage current to cause breakdown in the lower field. The lower leakage current in the flat region (hopping conduction region) of the film deposited at higher substrate temperature can be explained by the fact that the barrier height for hopping conduction is maximum in film deposited at 650℃.
From the TEM analysis, It was observed that BST films on Pt/$SiO_2$/Si have columnar structure, though BST films deposited on Pt/Ta/$SiO_2$/Si have grained structure. With increasing film thickness, the grain size of the BST films deposited on Pt/$SiO_2$/Si gradually increased. Dielectric constant of the BST films deposited at 600℃ also increased with increasing grain size. It was measured that the dielectric constant decreases from 758 to 348 with the grain size of 82-32nm in the BST films. It was revealed The abrupt decrease of dielectric constant in thinner film(below 60nm) is influenced by the grain size of BST thin films, in addition to by a low dielectric layer formed at the initial deposition stage. BST thin film with the thickness of 50nm deposited at 600℃ has a dielectric constant of 348 and a leakage current density of about $2.0×10^{-7}A/㎠$ at 1.5V. In the current-voltage curve of thinner BST film, higher leakage current density in the low bias region and narrower flat region were obtained.
