Possible routes for cluster growth and particle formation in RF silane discharges

After having distinguished five successive steps in the temporal evolution of a powder-forming SiH(4) radiofrequency glow discharge, we examine the Initial mechanism by which silicon clusters start growing up to the point where they suddenly aggregate into multiply charged particles and modify the discharge regime. This 'induction' period can be much longer than the diffusion time of positive ions and neutral radicals, which implies that cluster growth involves negative ions (anions). We provide a review of basic data concerning anions in SiH(4) plasmas and analyse mass spectrometric data showing that anion- molecule reactions Si(n)H(2n+1)(-) + SiH(4) -> S(n+1)H(2n+3)(-) + H(2) at relatively low rate (about 10(-12) cm(3) s(-1)), and fast exothermic anion-radical reactions Si(n)H(m)(-) + SiH(m) -> Si(n+1)H(m+m'-2q)(-) + qH(2) at Langevin rates (about 10(-9) cm(3) s(-1)), initiate clustering. The effective anion lifetime involves a succession of dissociative attachment to SiH(4), detachment or recombination, and attachment to neutral radicals or clusters competing with diffusion out of the plasma. Anion-molecule and anion-radical cluster reactions at Langevin rates probably dominate the cluster growth kinetics below 100 Si atoms whereas anion-neutral and neutral-neutral condensation at size-scaling collision rates govern the subsequent homogenous nucleation regime. At the end of the nucleation period (up to 10(4) Si atoms) the fraction of singly charged clusters can reach 50%. The reduction of powder formation upon gas heating is attributed to a decrease of the rate of nondissociative attachment to radicals and neutral clusters.