Browsing by Subject "Dusty plasma"
Now showing 1 - 3 of 3
Results Per Page
Sort Options
Item Charge and energy interactions between nanoparticles and low pressure plasmas.(2010-05) Galli, FedericoIn this work, the interactions between low-pressure plasmas and nanoparticles are studied with numerical models aimed at understanding the phenomena that affect the nanoparticles charge, charge distribution, heating, and crystallization dinamycs. At the same time other phenomena that affect the plasma properties resulting from the presence of nanoparticles are also studied: they include the power-coupling to the plasmas, the ion energy distribution and the electron energy distribution. An analytical model predicting the nano-particle charge and temperature distributions in a low pressure plasma is developed. The model includes the effect of collisions between ions and neutrals in proximity of the particles. In agreement with experimental evidence for pressures of a few Torr a charge distribution that is less negative than the prediction from the collisionless orbital motion limited theory is obtained. Under similar plasma conditions an enhanced ion current to the particle is found. Ion-electron recombination at the particle surface, together with other particle heating and cooling mechanisms typical of silane-argon plasmas, is included in a particle heating model which predicts the nano-particle temperature. The effect of plasma parameters on the nano-particle temperature distribution is discussed and the predictive power of the model is demonstrated against experimental evidence of temperature induced crystallization of silicon nano-particles. The power coupled to the plasma is measured together with the impedance nature of the plasma, in the case of a pristine and dusty plasma. Nanoparticles are shown to strongly affect the electrical properties of the plasma, resulting in a much more resistive discharge. A study of the ion energy distribution of ions impinging the sruface of nanoparticles is carried out and shows that ion-neutral collisions in proximity of the surface of the nanoparticle not only affects the particle charge but also the average energy of ions bombarding the particle surface. Finally the presence of nanaparticle in the plasma and their ability to selectively interact with electrons in a specific energy range is studied to the extent of investigating the effects of the presence of particles on the electron energy distribution of electrons.Item Numerical modeling of plasmas in which nanoparticles nucleate and grow(2012-10) Agarwal, PulkitDusty plasmas refer to a broad category of plasmas. Plasmas such as argon-silane plasmas in which particles nucleate and grow are widely used in semiconductor processing and nanoparticle manufacturing. In such dusty plasmas, the plasma and the dust particles are strongly coupled to each other. This means that the presence of dust particles significantly affects the plasma properties and vice versa. Therefore such plasmas are highly complex and they involve several interesting phenomena like nucleation, growth, coagulation, charging and transport. Dusty plasma afterglow is equally complex and important. Especially, residual charge on dust particles carries special significance in several industrial and laboratory situations and it has not been well understood. A 1D numerical model was developed of a low-pressure capacitively-coupled plasma in which nanoparticles nucleate and grow. Polydispersity of particle size distributions can be important in such plasmas. Sectional method, which is well known in aerosol literature, was used to model the evolving particle size and charge distribution. The numerical model is transient and one-dimensional and self consistently accounts for nucleation, growth, coagulation, charging and transport of dust particles and their effect on plasma properties. Nucleation and surface growth rates were treated as input parameters. Results were presented in terms of particle size and charge distribution with an emphasis on importance of polydispersity in particle growth and dynamics. Results of numerical model were compared with experimental measurements of light scattering and light emission from plasma. Reasonable qualitative agreement was found with some discrepancies. Pulsed dusty plasma can be important for controlling particle production and/or unwanted particle deposition. In this case, it is important to understand the behavior of the particle cloud during the afterglow following plasma turn-off. Numerical model was modified to self consistently simulate the dynamics and charging of particles during afterglow. It was found that dusty plasma afterglow is dominated by different time scales for electron and ion dynamics. Particle size and charge distribution changes significantly during the afterglow. Finally, a simplified chemistry model was included in dusty plasma numerical model to simulate the dynamics of argon-silane dusty plasma. The chemistry model treats silane dissociation and reactions of silicon hydrides containing up to two silicon atoms. The nucleation rate is equated to rate of formation of anions containing two Si atoms, and a heterogeneous reaction model is used to model particle surface growth. Evolution of particle size and concentration is explained and the importance of variable surface growth rate and nucleation rate is discussed.Item Studies of Submicrometer Particle Charging, Trapping, and Agglomeration in Steady and Transient Plasmas(2022-02) Ono, ToshisatoThe 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.