The Onset of Silicon Nanoparticle Nucleation in a Low Temperature Plasma*

Controlling the nucleation and growth of nanoparticles (NPs) in low temperature plasma systems is imperative for controlling the NP size distribution; and for some applications, such as chemical vapor deposition and polysilicon production, for preventing particle contamination. Although the mechanisms behind silicon NP nucleation and growth have been well studied, it is unclear how controllable system parameters, e.g., pressure and system geometry, affect the onset of NP nucleation. The diffusion of reactive silane species, particularly SiH₂ and SiH₃, is expected to significantly affect the feed concentration of silane required to nucleate silicon NPs (the nucleation onset concentration) due to losses at the reactor walls. Additionally, plasma parameters are expected to be significant as they are known to affect the size and structure of synthesized nanoparticles. In this work, the nucleation onset concentration was determined as a function of system pressure, applied plasma power, and reactor diameter for a tubular flow-through radiofrequency (RF) plasma generated using Ar/He/SiH₄ gas mixtures. A quartz crystal microbalance (QCM) impactor was used to measure the total aerosol mass density after the plasma and thereby identify the nucleation onset. A comparison of the experimental results to a 2D multi-physics model will also be provided.