Chemical Vapor Deposition of TiN from Tetrakis(dimethylamido)titanium and Ammonia: Kinetics and Mechanistic Studies of the Gas-Phase Chemistry

The gas-phase kinetics of the reaction of tetrakis(dimethylamido)titanium (Ti(NMe 2 ) 4 ) with NH 3 have been measured using a flow tube reactor and FTIR spectrometer. Ti(NMe 2 ) 4 reacts rapidly with NH 3 in a transamination reaction to form HNMe 2 as a direct product. The bimolecular rate constant for the reaction of Ti(NMe 2 ) 4 with NH 3 at 24 °C is k = (1.2 ± 0.2) × 10 - 16 cm 3 molecules - 1 s - 1 . A primary kinetic isotope effect of k H / k D = 2.6 ± 0.4 is observed with ND 3 indicating that cleavage of an N−H bond is the rate limiting step. Therefore the rate constant is assigned to the initial transamination reaction with NH 3 . The temperature dependence of the rate constant gives activation parameters of log(𝒜) = −10.0 ± 0.2 (Δ S † = −19 cal/(mol K)) and E a = 8.1 ± 0.1 kcal/mol (Δ H † = 6.9 ± 0.1 kcal/mol). When excess HNMe 2 is added to the gas flow, the reaction rate is strongly suppressed. This is evidence for a reversible initial transamination reaction: Ti(NMe 2 ) 4 + NH 3 ⇄ (Me 2 N) 3 Ti−NH 2 + HNMe 2 . The proposed mechanism for subsequent reaction is elimination of HNMe 2: (Me 2 N) 3 Ti−NH 2 → (Me 2 N) 2 Ti NH + HNMe 2 . From the dependence of the observed rate constant on HNMe 2, the branching ratio is obtained for the above elimination reaction versus reaction with HNMe 2: (Me 2 N) 3 Ti−NH 2 + HNMe 2 → Ti(NMe 2 ) 4 + NH 3 . The relevance of these results to the chemical vapor deposition of TiN by this chemistry is discussed.