Dust particle(s) (as) diagnostics in plasmas

Abstract

Complex plasmas are plasmas with dust particles or liquid droplets suspended in them. Under certain conditions dust particles spontaneously emerge in reactive plasmas. These plasmas, containing for instance fluorocarbon, hydrocarbon or silane molecules, are regularly named dusty plasmas. They have a broad range of applications in, for example, astrophysics (planetary rings, nebulae, comet tails, etc.). Also, the presence of dust particles in plasmas has considerable implications on plasma processes in the semiconductor and solar cell industry. Here, the properties of the plasma-processed surfaces can seriously be affected either negatively or positively; while contamination of semiconductor structures with dust particles might have a destructive effect, the incorporation of nanocrystals in deposited layers for solar cells can result in an increased efficiency and enhanced long-term stability of these devices. Another large application area is the presence of dust particles in fusion plasmas. In the plasma in these reactors, wall material can nucleate, forming dust particles that influence the plasma operation negatively. In all these applications, especially the dependence of dust particle formation on the gas temperature Tg is still not understood in full detail. In another field of research, dust particles (of several micrometers in size) are not created inside the plasma, but are injected from outside. These plasma configurations, often named complex plasmas, are regularly used for fundamental research. The highly negatively charged microparticles have the ability to form Coulomb crystals, making them representative macroscopic models for solid-state crystals and for phase transitions. Also, these charged particles can be confined within the space charge layer at the border of the discharge – the plasma sheath – where they can be used as electrostatic probes to measure the local electric field strength. In this thesis, two research lines are defined. In the first research line, we elaborate on the processes responsible for the formation and the growth of dust particles in low pressure hydrocarbon-containing (acetylene or methane) radiofrequency (RF) discharges. In acetylene, the first step in the dust particle formation process – the formation of negative ions – is studied by monitoring the time-evolution of both the electron density and the density of the first-formed and smallest negative ions (C2H- and/or H2CC-). Diagnostics used are microwave cavity resonance spectroscopy (MCRS) and laser-induced photodetachment. Both the formation and the destruction mechanisms of negative ions are found to be increasing functions of the gas temperature, the latter more so than the former. Since the rate of the dominant anion destruction channel (polymerization with neutral ground-state C2H2 precursor molecules) is independent of Tg, this is an indication for an increased density of (vibrationally) excited C2H??2 molecules at higher temperatures, enhancing polymerization rates. The coagulation onset time and the particle growth rate are studied in acetylene- and methane-containing discharges as function of the gas temperature as well. Used diagnostics are monitoring the time-evolution of the phase angle between the RF voltage and current, laser light scattering and, again, MCRS. In both gases, the coagulation onset time increases with decreasing pressure and increasing gas temperature. This behavior can be explained in terms of a changing gas density and a temperature dependent diffusion coefficient for nanoparticles. The growth rate of the dust particles decreases monotonically as function of Tg for methane containing discharges, and shows a maximum around Tg=65 0C for the acetylene case. A possible explanation for this behavior is a competition between increasing radical velocity towards the particle’s surface, decreasing radical sticking coefficients and decreasing particle densities, all at elevated temperatures. In the second research line, microparticles are injected into a collisional argon RF discharge and serve as electrostatic probes in the sheath region of the plasma. Probed parameters are the electric field strength and the particle charge, both obtained spatially resolved from measurements of the particle equilibrium position under varying apparent gravity conditions. Similar experiments are performed under hypergravity conditions (induced by a centrifuge) and under microgravity conditions (during parabolic flights). The results from the hypergravity experiments show a slightly non-linear electric field profile and indicate the presence of a maximum in the particle charge around 5.0 mm above the RF electrode. The microgravity experiments clearly show the transition region – the pre-sheath – separating the plasma sheath from the quasi-neutral plasma bulk, for instance by a kink in the electric field profile, and by dramatically changing ion and electron densities. Hence, we experimentally verify the existence of the pre-sheath region in RF discharges. From the measurements in concert with a simplified analytical sheath model, next to profiles for the electric field and the particle charge, profiles for the electron and ion density and for the directed ion velocity are obtained as well. The value of the directed velocity with which the ions enter the sheath region appears to be lower than the Bohm velocity, but still superthermal. Furthermore, from the experiments during the parabolic flights, interesting behavior is observed in terms of particle confinement in the absence of the apparent gravitational force. At pressures lower than ??0.2 mbar, the microparticles are lost from the discharge, while at higher pressures the particles remain confined within the sheath / pre-sheath. This behavior is explained by enhanced ion drag forces at higher pressures (increased ion density)