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Effects of Precursors and Substrate Materials on Microstructure, Dielectric Properties and Step Coverage of (Ba,Sr)TiO3 Films Grown by Metalorganic Chemical Vapor Deposition Y. Gao, S. He,(a) M. Engelhard, A. S. Lea, P. Alluri,(b,c) J. Finder,(b,c) B. Melnick,(b,c) and R. L. Hance(b,c) Supported by OBER EMSL Operations and Motorola, Inc. (a) Student. (b) EMSL User. (c) Materials Technological Laboratories, Motorola Inc., Austin. (Ba,Sr)TiO3 (BST), a perovskite material, is a promising high dielectric constant material for dynamic random-access memory (DRAM) capacitors and gate oxides for future generations because of its unique combination of high dielectric constant, low dielectric loss, low leakage current density, and good thermal stability. Recently, we have studied the precursor chemistry and film-formation mechanisms during BST MOCVD growth. The precursor chemistry was found to be different on oxide-covered Pt(111) surface as compared to the clean Pt(111) surface. In an oxygen ambient, there were at least four different reaction processes that involved the removal of carbon from the precursor ligands on oxide-covered Pt(111). In two of these, gas phase oxygen was incorporated into the oxidative products. In contrast, one carbon removing reaction was observed on the clean Pt(111) surface. Isotopic labeling experiments revealed that the oxygen in the BST films originated from both the gas phase oxidants (18O) and the precursor ligands (16O). The ligand substitution by gas phase O2 played a more prominent role in the film formation at lower temperatures. On the other hand, the reactive oxygen radicals produced by microwave plasma involved more in breaking the O-C bonds than in substituting the precursor ligands for the film formation. Use of the 50%18O2-50%N216O mixture results in a reduction of 18O incorporation in the BST films, indicative of the direct involvement of N2O in the film-formation reactions. These results have shed considerable light on understanding the origin of conformal step coverage of BST thin films on 3D structures. In this study, we have focused on the effects of precursors and substrate materials on film microstructure and crystallinity, conformal step coverage, and dielectric properties. It was found that precursor reactivities have no apparent effect on the microstructure, surface morphology, and dielectric properties of the stoichiometric BST films, but show strong influence on the step coverage of BST films on 3D Pt electrodes. These structural and electrical properties of the BST films strongly depend on the film composition, substrate material, and growth temperature. In general, the BST films grown on Pt exhibit better crystalline quality, surface smoothness, and dielectric properties compared to those grown on Ir under the optimal growth conditions. Typical dielectric constant and dielectric loss under optimal growth conditions are in the range of 300-600 and 0.02-0.05, respectively, measured at 104 Hz for 400- to 800-Å-thick BST films grown at 650° C using an O2/N2O mixture. The dielectric constant decreases as the film composition deviates from the stoichiometry. Plasma treatment of the Pt surface before deposition increases the dielectric constants. Figure 4.4 shows typical XRD patterns for BST films grown on Ir/Si and Pt/Si substrates under the same growth conditions. We have deposited BST films on both Pt(111) and Ir(111) using three different kinds of BST precursors. All BST films on Pt(111) are highly (001) textured (Figure 4.4a), whereas the films on Ir(111) typically show additional growth orientations such as (111) and (110) (Figure 4.4b). The results indicate that nucleation and film growth are dominated by the (001) oriented nuclei on highly (111) textured Pt substrates, while homogeneous nucleation and grain growth occur on Ir/Si substrates. Thus, the growth texture is not sensitive to the type of precursors, but to the substrate materials and film composition. Figure 4.4. X-ray diffraction patterns of stoichiometric BST films grown on Ir (a [left]) and on Pt (b [right]) at 650° C. Although the kind of precursors have no apparent effect on the film microstructure and dielectric properties, it strongly influences the step coverage of the BST films on 3D electrodes (Figure 4.5). The good step coverage (>80%) has been achieved for the BST films grown in a N2O/O2 mixture using the BST precursors that exhibit similar reactivities (Figure 4.5). Figure 4.5. SEM image of a BST film grown at 650°C in a N2O/O2 mixture, showing ~80% step coverage. As discussed above, the film-formation reactions were dominated by both oxidation and thermal decomposition of the precursors under these conditions. These two completing reactions, together with the effect of N2O on the reaction rate, may be attributed to the good step coverage and high crystallinity. In addition, we found that the step coverage is much more sensitive to the precursor properties than to the growth conditions. William R. Wiley Environmental Molecular Sciences Laboratory Feedback: webmaster@emsl.pnl.gov Revised: June 12, 2001 Security & Privacy PNNL-13147