Thermochemistry and Kinetics of Gas-Phase Reactions Relevant to the CVD of Coatings: New Data for Process Models
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the substrate. At low temperatures (800 K), where gas-phase reactions are very slow, growth rates are governed by a slow surface reaction and are independent of the rate of transport to the surface. At intermediate temperatures (1000 K), gas-phase reactions begin to occur, forming reactive intermediates such as SiH 2 that increase the growth rate substantially. Decreasing the residence time in the gas-phase by increasing the disk rotation rate decreases the concentration of these species, thus causing the deposition rate to decrease. At the highest temperatures (1300 K), gas-phase reactions are so fast that the growth rate becomes mass-transport limited, resulting in a deposition rate that increases with increasing rotation speed. A second example is provided by deposition of titanium nitride (TiN) from tetrakisdimethylamidotitanium (TDMAT). 2 In the absence of reactant ammonia, a slow reaction between TDMAT and the surface leads to excellent step coverage; however, the deposits contain high concentrations of carbon, which increase the resistance of the film to unacceptable levels. When ammonia is added to the gas mixture, a transamination reaction occurs that removes the N(CH 3)2 groups from the precursor and replaces them with NH 2 . The reactivity of this species with the surface is apparently much greater, leading to poor step coverage. However, because the precursor no longer contains carbon, the purity of the films is improved. Finally, growth of diamond via CVD is known to depend on the reaction of gas-phase methyl and hydrogen radicals with the surface. 3 While reaction of methyl can lead to diamond growth, it can also produce graphite. The role of hydrogen atoms is to remove the sites leading to graphite formation and allow diamond growth to occur. The concentration of both species depends on the gas-phase residence time and temperature profiles, as well as on the hydrocarbon precursor that is used. 121 Mat. Res. Soc. Symp. Proc. Vol. 555 01999 Materials Research Society
From these examples it is clear that the potential for gas-phase chemistry in CVD systems should not be ignored during model development. Unfortunately, many of the precursor chemistries used in CVD are poorly understood, if at all. Basic thermodynamic data, particularly for compounds containing organic ligands, are often lacking in standard references. 4 ,5 It is thus up to the individual investigator to estimate, virtually from scratch, both molecular heats of formation and rate constants for the reactions that may be occurring. Since chemistry at high temperatures can be quite complex, involving dozens or even hundreds of reactions, this task can be quite daunting. Fortunately, experimental and theoretical tools are available that make it possible to measure and/or predict the thermochemistry and kinetics of these reactions. The development of these tools is largely due the extensive scientific interest in combustion, pollution, and atmospheric problems, as well as in CVD. It is thus possible to assess, in at least a semi-quantitative
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