Surface Reaction Mechanisms in the Chemical Vapor Deposition of (Ba,Sr)TiO 3 Films
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SURFACE REACTION MECHANISMS IN THE CHEMICAL VAPOR DEPOSITION OF (Ba,Sr)TiO3 FILMS M. Yamamuka, T. Kawahara, M. Tarutani, T. Horikawa, T. Oomori, and T. Shibano Advanced Technology R&D Center, Mitsubishi Electric Corporation 8-1-1 Tsukaguchi-Honmachi, Amagasaki, Hyogo 661-8661, Japan ABSTRACT Surface reaction mechanisms in the chemical vapor deposition of (Ba,Sr)TiO3 [BST] films were studied by investigating the effects of O2 gas and source supply ratios on the characteristics of atomic incorporation rates. The atomic incorporation rates of Ba, Sr, and Ti increased with increasing incident flux of each source material, and then the values of the atomic incorporation rates became saturated. The saturated values increased monotonously with increasing O2 gas flow rate, in a range where atomic incorporation reactions might be controlled by the kinetics on the film surface (kinetically limited). Accordingly, O2 gas may effect the behavior of film precursors on BST film growth surfaces. From this, we assumed a CVD model, where the precursors are transported onto the film surface and adsorbed on adsorptive sites, and where the O2 gas has an effect on the formation of the adsorptive sites. With this model, atomic incorporation rates and overall sticking coefficients for the CVD of BST films were numerically simulated, and were in good agreement with experimental results for several O2 flow rates and source supply ratios. INTRODUCTION Three-dimensional (3D) stacked cells with high-dielectric-constant (Ba,Sr)TiO3 [BST] film have been proposed for Gbit-scale memories and beyond. Capacitors with such stacked cells require the BST film prepared by a chemical vapor deposition (CVD) method to have conformal step coverage. For this purpose, various apparatuses and processes for preparing BST thin films by liquid-source CVD have been developed to fabricate stacked capacitors in Gbit-scale integration [1-2]. The source materials have usually been bis (dipivaloylmethanato) barium [Ba(DPM)2], bis (dipivaloylmethanato) strontium [Sr(DPM)2], and titanium bis (isopropoxyl) bis (dipivaloylmethanato) [Ti(i-PrO)2(DPM)2] dissolved into organic solvent tetrahydrofuran (THF:C4H8O). There is a report of conformal trench coverage values of >70% with a trench having an aspect ratio of 3.3 for CVD-BST films [3]. In order to improve the step coverage, it is important to understand the chemistry of precursors on the film surfaces. Accordingly, we measured the atomic densities contained in CVD-BST films using an X-ray fluorescence (XRF) method, and investigated characteristics of the atomic incorporation rates of Ba, Sr, and Ti at several source gas supply ratios at different substrate temperatures. Moreover, we discussed a CVD model for BST film under the assumption of the Langmuir isotherm adsorbing mechanism. In the model, adsorptive sites were assumed to be formed on the BST growth film surface. This assumption made it possible for film precursors to be adsorbed on the sites with their own sticking coefficients. HerH WKH VWLFNLQJFRHIILFLHQWV
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