Spatiotemporal powder formation and trapping in radio frequency silane plasmas using two-dimensional polarization-sensitive laser scattering

Abstract

Powder formation studies in deposition plasmas are motivated by the need to reduce contamination in plasma and films. Models of the forces acting upon particles in radio frequency (rf) discharges suffer from a lack of quantitative experimental data to which to compare in the case of silane-containing plasmas. In this work, a cross section of the parallel-plate capacitor discharge is illuminated by a polarized beam-expanded laser, and global spatiotemporal scattered light and extinction are recorded by charge-coupled device cameras. Spatially regular periodic bright/dark zones due to constructive/destructive Mie interference are visible over large regions of the powder layers, which shows the uniform nature of particle growth in silane plasmas. For particles trapped in an argon plasma, and for steady-state conditions in silane, spatial size segregation is demonstrated by fringes that reverse according to the polarization of scattered light. The method allows a self-consistent estimation of particle size and number density throughout the discharge volume from which strong particle Coulomb coupling (Γ≳40) is suggested as influencing powder dynamics. An adjustment to the plasma emission profile must be made for extinction by the powder. This global diagnostic improves the understanding of particle growth and dynamics in silane rf discharges and provides experimental input for testing the validity of models.