Optical diagnostics of dusty plasmas

Abstract

The central topic of this thesis is dusty plasmas, in which particles are generated or injected. Such plasmas, when ignited in silane-based gas mixtures, are widely used in the semiconductor industry for depositing silicon layers (amorphous, micro-crystalline or polymorphous). These layers have applications in at panel displays, sensors, and solar cells for instance. The inclusion of nano-crystallites in the amorphous silicon layer produces cells with enhanced properties but calls at the same time for a better comprehension and control of the particles' formation and growth. The role played by silicon-based radical species in these processes more particularly prompts detailed studies. Dusty plasmas are also a ??eld of enduring interest to the astrophysics community. The interstellar medium can be simulated in a laboratory plasma to identify the carbon-based molecular complexes (Polycyclic Aromatic Hydrocarbons or PAHs) whose ions are thought to be responsible for unidenti??ed emission and absorption bands seen in the spectra of starlight. This thesis covers some aspects of both industry-oriented and astrophysical dusty plasmas. The experimental study on silane-based plasmas includes optical measurements performed in emission, absorption, and by analyzing the light scattered by particles grown in-situ. The negative charge acquired by the particles while immersed in the plasma disturbs their dynamics but also the electrical properties of the discharge. Based on the monitoring of the plasma impedance, it is shown that the plasma is a??ected by the particles' presence, independently from the nature of the silane carrier gas. Optical emission spectroscopy performed on SiH, H?? and H2 excited states indicates that the silane dissociation occurs in the vicinity of the RF-powered electrode. A Fourier Transform Infrared (FTIR) time-dependent analysis of the silane consumption after plasma ignition demonstrates that the silane dissociation is actually a slow but continuous process which increases with the occurrence of dust particles and leads to a dissociation degree between 80 and 97 %. Short lifetime radicals resulting from dissociation are suspected to play some role in the particles' formation and ??rst growth stages near the RF electrode. In order to identify those radicals and determine their spatio-temporal properties, Cavity Ring-Down Spectroscopy (CRDS) in the infrared wavelength range is used for its ability to measure absolute densities of transient gasphase reactive species. The CRDS in this work is based on a novel detuning method which does not require switching the laser o?? or interrupting the beam. The agreement of the detuning's numerical modeling with experimental results assesses the technique's relevance and shows it is well suited to further investigation of dusty silane plasmas. After the particles are formed in the plasma, a subtile balance of axial and radial forces traps them in the plasma bulk, mainly at the sheath boundaries, as it is experimentally proven by means of laser light scattering. FTIR measurements performed on particles growing in the plasma shows they generate solid-state absorption features at wavenumbers which are characteristic of SiH, SiH2 and SiH3 infrared active bonds. FTIR spectra also reveal that dust particles trapped in the plasma apparently get oxidized by residual water present in the vacuum system. However, particles are expelled from the discharge and carried away by the neutral gas ow when the forces are not balanced any more. This explains the scattering of laser light, that can be observed outside the plasma chamber. In addition to the above work, an experimental and theoretical study is performed on the nature, the structure and the behavior of a plasma generated in a supersonically expanded gas. That device, also referred to as pulsed discharge nozzle (PDN), has been developed to investigate the absorption spectroscopy of interstellar dust analogs in an astrophysically relevant medium. The plasma is de??ned as a glow discharge operating in an abnormal regime, and its structure is reduced to a negative glow and dark spaces at the electrodes. Cavity Ring-Down Spectroscopy is used to probe the plasma in pristine argon as it expands into the vacuum chamber. The electron temperature is found to be very low and it is shown that the zone probed by the laser is not in local thermal equilibrium. This suggests that few electrons are transported out of the inter-electrode region along with the expansion. Con??rmation is given by two-dimensional numerical modeling of the discharge generated in the PDN. The most active region is indeed located near the cathode where argon is primarily ionized. Argon metastables, produced at both electrodes, are not a??ected by the electric ??eld across the PDN and are thus transported downstream along with the neutral gas ow. This makes the PDN a particularly e??cient source of cold metastable argon atoms. The latter are capable of ionizing the PAHs molecules, normally fed into the PDN device, with a limited but non-negligible fragmentation.