HOW TO GROW LARGE CLUSTERS FROM SIXDY+ IONS IN SILANE OR DISILANE - WATER THEM

Growth of large cationic clusters is observed in real time for subsilane and subdisilane cations in the presence of silane/disilane-water mixtures. SiD0-3+ and Si2D0-6+ are created by electron impact in the trapped ion cell of a Fourier transform mass spectrometer and their sequential clustering reactions with 5% water: 95% silane/disilane are monitored for up to 80 s at total pressures of 10(-7)-10(-5) Torr. Formation of SixDyO(z)+ clusters out to at least 450 amu in silane and 650 amu in disilane can be seen on the available experimental time scales. The early portion of the sequence leading to large clusters has been elucidated for silane. Amazingly, of the possible subsilane cations, only SiD+ reacts with silane and water to form increasingly larger cluster sizes. Reactions of the other subsilane cations, SiD0,2-3+, do not continue without apparent limit. Initial growth of SiD+ proceeds in a highly specific fashion involving the formation of two critical doorway ions, Si4D7+ followed by Si4D7O+. The growth pattern then fans out to include numerous alternating and parallel reactions with both SiD4 and D2O. Several general features of the growth reactions are seen. Reactions with SiD4 are noticeably slower than reactions with D2O. Cluster growth by bimolecular reaction with SiD4 and D2O occurs by addition of SiD2 and addition of an oxygen atom, respectively, accompanied by elimination of D2. Loss of additional molecules of D2 sometimes occurs, particularly as clustering proceeds to large sizes. Cluster growth by termolecular attachment of SiD4 or D2O is also seen. This process results in the formation of SixDyOz+ complexes with SiD4 and D2O that appear to serve as important intermediates which enhance cluster growth rates as the total pressure is increased. Sequential clustering without apparent limit is only observed for subsilane and subdisilane cations with silane and disilane when water is present. On this basis, it is proposed that low levels of water contamination can provide a key ingredient for the chemistry which leads to the formation of the hydrogenated silicon particles found ubiquitously in silane plasmas.