Stagnation Flow Reactor Investigation of Tin Oxide CVD from Monobutyltin Trichloride

Abstract

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,...

Computational fluid dynamic modeling of tin oxide deposition in an impinging chemical vapor deposition reactor

“…Such a high deposition rate as well as the experimental observation of the wave shape deposition profile (will be discussed later) imply that the deposition process of SnO 2 from MBTC might be mainly limited by the flow behavior as well as mass transport in the coating zone and that the effect of reaction kinetics is less important. This assumption is further substantiated by the experimental measurement of tin oxide deposition rate at different temperatures [7,8]. As a result, the CFD modeling of on-line APCVD coating on glass can be done with reasonable accuracy without a highly developed reaction mechanism and kinetic data.…”

“…The numerical simulation based on CFD is able to provide quantitative information of flow behavior and species transport, but typically relies to a large extent on the thermodynamic data, reaction mechanism, and transport properties. Monobutyltin trichloride (or MBTC) used for SnO 2 deposition in the glass industry, which is of interest in the current work, has very limited data reported in the literature [6][7][8][9][10][11]. While it is experimentally shown that the reaction of MBTC with oxygen is accelerated in the presence of water, the function of water in the decomposition and/or oxidation of MBTC, is not fully understood.…”

“…The current model is developed using CFD with an impinging flow geometry, and explicitly accounts for homogenous reaction in the gas phase, heterogeneous reaction on the glass surface, thermal effect of the impinging jet on the glass, and impinging flow characteristics in the confined coating zone. The reaction kinetics are based on modeling of deposition rates measured in a stagnation-flow reactor [7,13]; certain kinetic parameters were modified to fit the experimental data. A comparison of CFD model predictions with experimental measurements shows that the experimentally observed spatial distribution in the deposition rate profile is successfully captured by the model.…”

X-ray photoelectron spectroscopy study on the chemistry involved in tin oxide film growth during chemical vapor deposition processes

“…Although reactions of tin tetrachloride with water and oxygen are relatively fast, Eley-Rideal mechanisms are not very likely since the loss of entropy and the minimal chance that molecules collide in the exact configuration needed for a reaction will give an extremely low pre-exponential factor, and so reaction rate. 9 The last group is represented by Chae et al 10,11 who interpreted their results also in terms of an Eley-Ridealtype of mechanism during his study on monobutyl tin chloride (MBTC, n-C 4 H 9 SnCl 3 ) with oxygen. However, for the deposition of MBTC with oxygen and water, the authors proposed that first a gas-phase complex of MBTC with water must be formed before the complex can adsorb at the surface and react with oxygen in an Eley-Rideal-type of reaction.…”

Film formation mechanism in glass lubrication by polymer latex dispersions

“…The oxide coating maintains the high pristine strength of glass [1,2]. Tin dioxide is currently used industrially for its high transparency to visible light and high hardness and is deposited from organometallic precursors such as tin tetrachloride (SnCl 4 ) [3,4] or monobutyltintrichloride (n-C 4 H 9 SnCl 3 ) [5][6][7][8][9]. The second one, called "cold-end coating", consists in a spray deposition of an organic water-based formulation (polyethylene, waxes, oleic acid …) at bottle temperatures between 80 °C and 160 °C [10,11].…”