Abstract

Multiphoton ionization mass spectrometry (MPI/MS) is used as a probe for methyl radical in the pyrolysis of methyl metals (trimethylaluminum, TMA; trimethylgallium, TMG), important molecules in chemical vapor deposition of semiconductors. In TMA pyrolysis, methyl radical is produced with an activation energy of 13 kcal/mole despite an aluminum–carbon bond strength of ~60 kcal/mole. No ethane or methane formation can be seen from any substrate. The proposed mechanism for the pyrolysis is that formation of a bond between aluminum and the electron donating surface catalyzes the cleavage of the aluminum–carbon bond, hence methyl production. On a nonconducting surface (unable to bond to aluminum), the deposition (pyrolysis) proceeds by uncatalyzed thermal cleavage until sufficient metal is deposited to permit the rapid (low activation energy) rate. The methane and ethane seen in commercial deposition reactors are then the products of secondary gas phase reactions and not surface products. The pyrolysis of TMG shows similar results. Implications for chemical vapor deposition are discussed.

© 1985 Optical Society of America