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Tin oxide (SnO 2 ) chemical vapor deposition from monobutyltin trichloride ͑MBTC͒ was performed in a stagnation flow reactor at reduced pressures ͑15-100 Torr͒ and temperatures of 573-923 K. Growth from MBTC/O 2 mixtures exhibits multiple activation energies, decreasing from 18.8 Ϯ 1.1 kcal mol Ϫ1 at temperatures р673 K to 11.4 Ϯ 1.1 kcal mol Ϫ1 at temperatures Ͼ673 K, indicating a change from one growth mechanism to another. Heterogeneous chemistry governs growth from these precursors at р673 K and most likely at higher temperatures as well, although previous work suggests that gas-phase MBTC pyrolysis/ oxidation may begin to contribute at the highest temperatures examined here. The pressure dependence of the MBTC/O 2 growth rate indicates an approach to the mass-transport limit at 923 K. In growth from MBTC/O 2 /H 2 O mixtures, deposition rates are dramatically higher than those produced by MBTC/O 2 ͑by a factor of over 20 at the lowest substrate temperatures͒. Potential mechanisms responsible for this increase in rate include both fully heterogeneous processes and rapid formation of a reactive MBTC-H 2 O complex. We conclude that multiple reaction pathways, including both gas-phase and surface processes, must be considered to develop a robust model of tin oxide growth from typical MBTC precursor mixtures.Tin oxide chemical vapor deposition ͑CVD͒ is an important industrial process used to deposit transparent conducting oxides on glass used in low-emissivity windows, solar cells, and flat-panel displays. On-line CVD coating over large-area substrates is routinely performed on float-glass production lines using both inorganic and organometallic tin precursors. 1-6 For on-line CVD coating, the process is difficult to optimize and control because of high line speeds that limit growth times to 1-3 s, leading to a requirement for fast growth rates and high volumetric gas flows. 6 This produces a thin thermal boundary layer over the substrate, and consequently, low reactant conversion efficiencies ͑as low as 10%͒. Stringent requirements for optical uniformity demand uniform growth rates across very large substrates ͑as much as 4 m wide͒. Although deposition temperature and reactant concentration have significant effects on growth rate, the need to minimize haze and to control coating color limits the ability to use these variables to enhance coating efficiency. 4 Because of the difficulty and cost of making changes in a full-scale manufacturing process, there is a clear need for kinetic models that account for the effects of process variables on deposition rate and reactant conversion efficiency to guide optimization efforts.One of the most important industrial CVD precursors is monobutyltin trichloride ͑MBTC͒. Development of a process model for SnO 2 deposition from MBTC requires extensive knowledge of chemical reaction rates, including the kinetics of MBTC thermal decomposition, oxidation, and hydrolysis, as well as SnO 2 growth rates. However, only a few studies of SnO 2 deposition using MBTC have been reported. 4,...

Global kinetic mechanisms and models are developed to describe the growth of tin oxide ͑SnO 2 ͒ films from monobutyltintrichloride ͑MBTC͒ by chemical vapor deposition. Several candidate mechanisms are examined. Deposition from MBTC + O 2 mixtures is best described by a mechanism in which MBTC reacts with an oxygen-covered surface, while deposition from MBTC + O 2 + H 2 O mixtures can be effectively predicted using a mechanism in which a gas-phase MBTC-H 2 O complex is formed, adsorbed by the surface, and reacts with gas-phase O 2 to form SnO 2 . Both models are based on new and previously reported growth-rate data obtained in a stagnation flow reactor ͑SFR͒ as a function of the inlet concentrations of MBTC, O 2 , and H 2 O, substrate temperature, inlet gas velocity, and total pressure. Model rate constants were obtained by fitting to SFR growth rates obtained at 25 Torr and relatively low MBTC concentrations ͑0.1-0.4 mol %͒. The predictions are within a factor of 2 of SFR data obtained at higher pressures ͑up to 100 Torr͒, as well as with results in the literature obtained at 1 atm, suggesting that the deposition mechanisms used capture the essential chemistry over a broad range of chemical vapor deposition conditions.

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