The formation of silicon nitride from trisilylamine and ammonia

Silane gas has been used for three decades as a precursor for plasma enhanced chemical vapour deposition processes but is unsustainable in the longer term due to the extremely hazardous nature of the compound. Alternative precursor materials have been proposed but have proved to be largely incompatible with the chemistry of the deposition process or the requirements of semiconductor process technology. One compound with the chemical and technological potential as a precursor for silicon nitride deposition is trisilylamine. Calculation of the Gibbs free energy change for the formation of silicon nitride from the reaction of trisilylamine and ammonia demonstrates that the reaction IS thermodynamically feasible as is the reaction involving silane with ammonia. The standard molar enthalpy of formation for trisilylamine was obtained from a semiempirical molecular orbital calculation while the standard molar entropy of formation was determined from spectroscopic data in the absence of a calorimetric value. Thermodynamic properties have been calculated for a range of aminated species using semi-empirical methods and entropy vs. molecular weight equations. These species are potential intermediates in a plasma discharge of trisilylamine and ammonia, with their successive combination leading to the deposition of a film of silicon nitride. Thermodynamic values for reactions involving the formation, propagation and termination of radical species of trisily lamine and ammonia have been determined and a mechanism is proposed for the deposition of silicon nitride films by plasma enhanced chemical vapour deposition. These results indicate that there is no thermodynamic barrier to the use of trisilylamine as a precursor with ammonia gas for the plasma enhanced deposition of silicon nitride films.

MPhil thesis.