Chem. Mater. 1991,3,953-960
953
Chemisorption of Trimethylaluminum and Ammonia on Silica: Mechanisms for the Formation of AI-N Bonds and the Elimination of Methyl Groups Bonded to Aluminum M. E. Bartram,* T. A. Michalske, J. W. Rogers, Jr.,t and T. M. Mayer Sandia National Laboratory, Albuquerque, New Mexico 87185-5800 Received April 1, 1991. Revised Manuscript Received July 22, 1991 Infrared spectroscopy and temperature-programmed desorption (TPD) have been used to study the formation and decomposition of a potential AlN precursor on the surface of silica. At 300 K, ammonia reacts with the monomethylaluminum surface complex produced by trimethylaluminum chemisorption to form a structure analogousto the TMANH3adduct known in the condensed phase. The surface adduct is stable in high vacuum and demonstrates that strong AI-N bonds can be produced on the surface at low temperatures. Additional AI-N bonds are formed when these species react to yield -NH2- (amino) groups that bridge between aluminum centers. Thermal decomposition of the methylaluminum:ND3 surface adduct leads to the desorption of CH3D below 600 K, suggesting that the formation of bridging amino groups is the coproduct of a reaction in which ammonia directly hydrogenates methyl groups bonded to aluminum. Collectively, these results imply that adjacent adducts decompose via an interadsorbate reaction mechanism that synergisticallydecreases the source of carbon contamination and promotes the formation of contiguous A1-N bonds on the surface at low temperatures. TPD results and the coverage dependence of bridging amino formation suggest that the extent of A1-N bonding is limited by the desorption of ammonia and by the proximity and orientation of the surface adduct.
Introduction Because of its attractive thermal, electronic, and mechanical properties,'"' AlN has attracted widespread attention in recent years. Applications in microelectronic2 and sensor techn~logies,~ in particular, require thin films of AlN deposited on substrata by using process conditions compatible with the rest of the circuit materials. For this reason, much of the recent emphasis has centered on the low-temperature deposition of A1N thin films. AlN has been deposited by a variety of sputtering and ion-beam deposition techniques." Thermal, plasma-assisted, and photoassisted chemical vapor deposition (CVD)processes have also been employed using reactants such as trimethylaluminum (TMA) (and other alkylaluminum compounds) and NH3,&11TMA and hydrazine,12J3AlBr3 and N2,7J4AlC13 and NH3,16and a number of molecular precursors that contain A1-N bonds.16J7 CVD processes offer the most flexibility in process design and potential control of material properties, but a great deal remains unknown about the chemical mechanisms of film growth by CVD. Kinetic studies of overall film growth rate using simple organoaluminum precursors suggest that scission of the A1-C bond is rate-limiting.8J3 However, A1N deposition kinetics have been shown to depend on the chemical nature of the nitrogen source. For example, hydrazine (N2H4)has been shown to increase the rate of deposition over ammonia (NH3).12J3To effectively utilize CVD techniques to nucleate and grow AlN films at low temperature (
