Studies of Submicrometer Particle Charging, Trapping, and Agglomeration in Steady and Transient Plasmas

Abstract

The particle dynamics in both steady-state and transient plasmas are of interest for applications in particle synthesis in plasmas, and in mitigation of contamination issues in semiconductor processing. Semiconductor processing plasmas need to be operated in an intermediate pressure regime (5-10 Torr) to achieve a higher manufacturing yield, leading to further particle defect issues by the formation or agglomeration of particles. Also, the presence of particles locally perturbs plasmas which lead to chamber arcing issues. This dissertation covers both experimental and theoretical studies of particle behavior in collisional sheaths in both steady-state and transient plasmas, as well as studies of thin-film synthesis by PECVD. The end results of the studies described in each chapter provide (1) a distinct modeling approach to quantify ion attachment and momentum transfer from ions to particles in a collisional sheath, (2) evidence of material-dependent particle trapping locations in steady-state plasmas, (3) the first direct observation of particle agglomeration facilitated by particle charging and decharging in pulsed plasmas, and (4) experimental and theoretical description of the link between plasma properties and synthesized thin-film properties by PECVD. The dimensionless ion attachment coefficients and dimensionless collection forces on negatively charged particles are calculated using ion trajectory models accounting for an external electric field in a collisional sheath, ion inertia, and finite ion mobility. By considering both ion inertia and finite ion mobility, results apply for ion transport from the fully-collisional regime into a regime of intermediate collisionality. We show that ion motion about a charged particle can be parameterized by the ion Stokes number and dimensionless electric field strength. Increasing the ion Stokes number is found to significantly decrease the dimensionless ion attachment coefficients and ion collection forces. We find the charge level is strongly sensitive to both field strength and pressure in the plasma sheath. Calculations are also used to demonstrate that the ion collection force can be sufficiently strong to trap particles close to both the top and bottom electrodes. Using laser light scattering (LLS), we examined the trapping of submicrometer metal oxide particles of 6 distinct material compositions in the sheath boundary of a CCP plasma in the intermediate pressure range. We find the trapping location shifts slightly farther from the top electrode with increasing material dielectric constant. This suggests that the ion drag force is influenced by particle material properties. Measured trapping locations are also compared to model predictions where particle charge levels and the ion drag force are calculated using expressions based on the ion trajectory calculations. Predicted ion densities required for trapping are a factor of 6-16 higher than calculated at the observed particle trapping locations. In total, our results confirm that submicrometer particle trapping occurs at the upper electrode of CCP reactors. Dispersed submicrometer particles are exposed to drastically varying ion and electron densities during both ignition and extinction of non-thermal plasma. RF pulsing enables examination of particle behavior in a transient afterglow and during plasma reignition. By progressively decreasing the pulsing frequency, agglomerates can be grown to highly non-spherical and millimeter scale agglomerates. We find that the pulsing frequencies which lead to agglomerated silica particles do not lead to equally large agglomerates with barium titanate particles, suggesting discharging rates are material dependent. Pulsed plasma systems can serve to control the extent of agglomeration in particulate systems, creating agglomerates multiple orders of magnitude large in length scale than the base primary particles, and this agglomeration is material dependent.