Decomposition of Trimethylaluminum on Si(100)

The mechanism of trimethylaluminum (TMAl) decomposition on a heated Si(100) substrate is studied by using temperature-programmed desorption, X-ray photoelectron spectroscopy, and electron energy loss spectroscopy. It is found that the TMAl adsorbs as dimers at low temperature. The dimers dissociate into monomers upon heating to 300-400 K. Further heating causes one of the methyl groups in the TMAl to react with a hydrogen on another methyl group, liberating methane. This leaves a CH2 group bound to the aluminum, which is seen clearly with EELS. Upon further heating, some of the remaining fragments undergo another intramolecular hydrogen shift, producing additional methane. The other fragments dissociate into aluminum atoms and CHX groups. The CHX groups are strongly bound on silicon. Hence, they decompose above 750 K, yielding adsorbed hydrogen and adsorbed carbon. The hydrogen desorbs between 750 and 850 K. The carbon remains bound to the silicon to high temperatures. These results show that carbon incorporation is an intrinsic part of the TMAl decomposition process on Si(100). Thus, TMAl would not be an appropriate source gas when one needs to produce films with very low carbon levels.