Mechanisms for Nanoparticle Synthesis and Charging in Nonthermal Plasmas

Abstract

Nanoparticle formation, charging, and transport in plasmas have been extensively studied due to increasing interest. In the semiconductor industry, dust particles are considered as defects and are therefore unwanted as they can damage electronic devices during plasma etching or chemical vapor deposition. Potential applications are emerging, including biomedicine or photovoltaics, and require unique particle size and material properties. With the advancement of new technologies, along with a better understanding of particle formation, it is possible to experimentally tailor particle properties as small as 1 nm in diameter. The aim of this thesis is to contribute to the understanding of the mechanisms underlying the formation of nanoparticles in plasmas. A two-dimensional model is developed to self-consistently examine nanoparticle formation, growth, charging, and transport in low-pressure, capacitively-coupled RF flowing plasmas. The experimental set-up modeled is a narrow quartz tube in which a gas mixture of argon-helium-silane flows. The silane dissociation is mostly produced by electron impact due to highly energetic electrons. The nanoparticle cloud is coupled to the plasma. The spatial evolution of the particle size distribution and charge distribution is presented. We show that nanoparticles are mostly negatively charged and pushed along the centerline of the discharge due the ambipolar electric field. However, particles are not trapped in the axial direction, which allows nanoparticles to grow as they flow through the tube. The model predicts the possibility of producing crystalline nanoparticles due to exothermic reactions (e.g., electron-ion recombination and hydrogen reactions) on nanoparticle surfaces. The charging of nanoparticles in plasmas plays a significant role in their growth mechanisms and transport. In a typical parallel plate plasma system, nanoparticles get negatively charged due to collisions with electrons and are trapped at the center of the discharge due to the ambipolar electric field. At small nanoparticle sizes (< 10 nm), the number of electrons that can coexist on a single particle is limited, referred to as particle charge limit. We studied the effect of particle charge limits on charge distributions in low-pressure nonthermal plasmas, by developing an analytical expression for the charge distribution and comparing it with a stochastic charging model. Particle charging plays a significant role in particle transport since charged particles respond to the electric field. Under typical plasma-enhanced chemical vapor deposition conditions, a certain fraction of particles can be neutral or positive and escape the plasma and deposit on the wafer. To better understand and control particle deposition, we studied the effects of ion collisionality with the background neutral gas, electron emission processes, electronegativity, and charge limit on charge distributions. Tailoring particle size and flux to a substrate is possible while using a pulsed plasma. Because particles are mostly negatively charged, they can be accelerated to the substrate when applying a positive DC bias when the plasma is OFF. Silicon particle formation, growth, and transport are discussed for the case of a parallel-plate capacitively-coupled RF pulsed plasma in hydrogen-silane. The transport of such particles in the afterglow is discussed, based on preliminary work exploring system behavior during the first cycle of such a pulsed plasma.