SILICON DEPOSITION FROM SILANE OR DISILANE IN A FLUIDIZED-BED .1. EXPERIMENTAL-STUDY

Fluidized-bed silicon deposition from silane constitutes an attractive alternative way to produce ultrapure silicon for solar-cell and microelectronic industries. Moreover, it allows the protection of bed particles from oxidation and corrosion. Studies available in the literature have proved the feasibility of this process, and pointed out its numerous advantages (high throughput, low-cost technology,...). However, two significant limiting problems exist: the parasitic formation of fines during experiments, and particles agglomeration when the initial concentration of silane exceeds a critical value. The first part of this article presents an experimental study about silicon deposition from monosilane and, for the first time, from disilane, in a fluidized-bed reactor. During experiments with monosilane, for inlet concentrations lower than 20% in nitrogen, silane conversion has always been quite complete, and fines formation limited. For higher concentrations, the fluidized bed has agglomerated systematically. Furthermore, for the first time, thermal perturbations of the fluidized bed have been put in evidence during all the runs, as soon as silane was introduced into the reactor. Such disturbances did not appear during disilane experiments, but fines were formed in larger amounts. Their zone of apparition was restricted to the coldest regions of the reactor. This has led us to think, in complete disagreement with literature opinions, that fines are formed from heterogeneous chemical reactions, on cold surfaces of the reactor. Moreover, thermal disturbances and agglomeration phenomena have been explained by an increase in particles cohesiveness, due to the presence of silane in the bed.