Showing papers on "Dusty plasma published in 1994"

Plasma crystal: Coulomb crystallization in a dusty plasma.

Abstract: A macroscopic Coulomb crystal of solid particles in a plasma has been observed Images of a cloud of $7\ensuremath{-}\ensuremath{\mu}m$ "dust" particles, which are charged and levitated in a weakly ionized argon plasma, reveal a hexagonal crystal structure The crystal is visible to the unaided eye The particles are cooled by neutral gas to 310 K, and their charge is $g9800e$, corresponding to a Coulomb coupling parameter $\ensuremath{\Gamma}g20 700$ For such a large $\ensuremath{\Gamma}$ value, strongly coupled plasma theory predicts that the particles should organize in a Coulomb solid, in agreement with our observations

Direct observation of Coulomb crystals and liquids in strongly coupled rf dusty plasmas.

Abstract: The strongly coupled dusty plasmas are formed by suspending negatively charged ${\mathrm{SiO}}_{2}$ fine particles with 10 \ensuremath{\mu}m diameter in weakly ionized rf Ar discharges. The Coulomb crystals and liquids are directly observed for the first time using an optical microscope. By properly controlling the system parameters, hexagonal, fcc and bcc crystal structures and solids with coexisting different crystal structures can be formed. Increasing the rf power causes the transition to the more disordered liquid state.

Cosmic dusty plasma

Abstract: Different types of photometric observations in the 1930s (Trumpler 1930; Stebbins et al 1 934; 1939) clearly showed that the dark "holes" in the Milky Way, observed by William Herschel almost 150 years earlier, were in fact re­ gions of heavy obscuration by cosmic dust. Continuing observations since then have established that dust is an almost ubiquitous component of the cosmic environment. Remote sensing of dust in the interstellar, circumstellar, inter­ planetary, circumplanetary, and cometary environments has, more recently been complemented by in-situ detections of the last three. Furthermore, the inference of the existence of very small grains (so-called VSGs with dimensions of 10100 A) in the interstellar medium (Puget & Leger 1989) as well as their in-situ detection in the environment of comet PlHalley (Sagdeev et al 1989) reinforces the reasonable expectation that the transition from gas to large dust particles in the cosmic environment is a continuous one through macromolecules, clusters, and VSGs. These dust grains are invariably immersed in ambient plasma and radiative environments. They must therefore be necessarily electrically charged and consequently coupled to the plasma through electric and magnetic fields, with the coupling becoming stronger as the grain size decreases. While any plasma containing such charged dust grains is often loosely referred to as a dusty plasma, there are different regimes characterized by the relative magnitudes of three characteristic length scales, namely the dust grain size a, the plasma Debye length AD, and the average intergrain distance d(� n;;1/3 , wherend is the dust number density). In general cosmic plasma en­ vironments that are contaminated by dust can be characterized by either of two conditions 1. a « AD < d or 2. a « d < AD. In the first case the dust may

Observation of Coulomb-Crystal Formation from Carbon Particles Grown in a Methane Plasma

Abstract: A Coulomb crystal was successfully formed as a result of the growth of spherical and monodisperse carbon particles suspended in a methane plasma. The crystal structure was confirmed to be hexagonal from top- and side-view photographs. The particle growth was monitored by Mie-scattering ellipsometry and correlated with the formation process of the Coulomb crystal. The liquid-to-solid phase transition occurred when particle diameter grew to 1.3 µm, and when the Wigner-Seitz radius was about 90 µm.

Charging of particles in a plasma

Abstract: Several models that predict the charge of particles in a plasma are reviewed. The simplest is based on orbit-limited probe theory. This basic model can be improved by adding several effects: charge reduction at high dust densities, electron emission, ion trapping and fluctuations. The charge is reduced at high dust densities, when a significant fraction of the charge in the plasma resides on the particles, depleting the plasma. Electron emission due to electron impact or ultraviolet exposure can cause a particle to have a positive charge, which has useful implications for plasma processing, since particles are confined in a discharge only if they have a negative charge. Ion trapping occurs due to ion-neutral collisions within the attractive Debye sphere of a negatively charged particle. Trapped ions reduce the net electric force on a particle. A particle's charge fluctuates because the currents collected from the plasma consist of discrete charges arriving at the particle at random intervals. The root mean square fractional fluctuation level varies as 0.5(N)- 12 / where (N)=(Q)/e is the mean number of electron charges on the particle.

Coulomb solids and low-frequency fluctuations in RF dusty plasmas

Abstract: The collective behaviour of weakly ionized RF dusty plasmas is investigated. Negatively charged SiO2 fine particles are generated and suspended in an SiH4/O2/Ar discharge. At high pressure (300 mTorr), the random fluctuations of the discharge and particle motions are suppressed. Coulomb solids with particle diameters from a few micrometres to a millimetre have been observed. At low pressure, the coupling between particles and the background plasma causes large-amplitude low-frequency fluctuations of the dusty plasma.

Propagation of waves in dusty plasmas with variable charges on dust particles

Abstract: The propagation of electromagnetic and Langmuir waves in multicomponent unmagnetized plasmas with dust particles is investigated. The effect of capture of plasma electrons and ions by the dust particles is taken into account. The recent theory proposed by Tsytovich and Havnes [Comments Plasma Phys. Controlled Fusion 15, 267 (1993)] to describe the kinetics of the capture process, as well as its perturbations, is further developed. The new wave damping due to this effect is calculated.

Overview of growth and behaviour of clusters and particles in plasmas

Abstract: A review is presented of the phenomena associated with particles in low pressure plasmas. Dust particles which are typically micrometers in diameter have been observed by laser light scattering in various low-pressure, radiofrequency-excited plasmas. Experiments have been designed so that the origin of the dust material is unambiguous and, to some extent, quantitative. The processes involved in the appearance of the mesoscopic dust particles are outlined and compared with our experimental observations. The source material and its required generation rate, nucleation, charging, growth mechanisms, growth rates, and saturation mechanisms are discussed. The mutual influences of dust and plasma, particularly the role of geometric and circuit boundary conditions in laboratory plasmas, are described.

High concentration effects in dusty plasmas

Abstract: Dust generation in plasma reactors used for PECVD is a general limiting effect which occurs when trying to obtain high deposition rates in the fabrication of thin films. In such dust-forming processes, for instance silane discharges, very high concentrations of submicrometre sized particulates are readily produced. The theoretical approach and the modelling of these dusty dense plasmas suggest that they have very peculiar properties with spectacular effects concerning the plasma equilibrium and the behaviour of the particulate cloud. Well characterized dusty dense plasma situations have been obtained in argon-silane or in pure argon RF discharges and experimental data obtained in these situations are reported here, in connection with the theoretical predictions. In terms of plasma properties the drastic modification of the free electron population, induced by the presence of the particles, is one of the most important results, with significant effects on the chemical equilibrium of the plasma. In terms of the particle cloud behaviour the strong electrostatic interaction between the negatively charged particulates is shown to be an order of magnitude higher than their kinetic energy and this particle cloud has to be described as a 'Coulomb liquid'. An overview of our experimental studies of these effects in a dusty dense plasma situation is given, including the most recent results.

A particle-in-cell simulation of dust charging and shielding in low pressure glow discharges

Abstract: The transport of particles ("dust") in low pressure electrical glow discharges is being studied in regard to its role in contaminating silicon wafers during plasma etching and deposition. Particles (10 s nm-/spl mu/m) negatively charge in glow discharges and, to first order, appear to be massively large negative ions around which sheaths develop. The forces on particles in plasmas include electrostatic (drift of charged particles in electric fields) and viscous ion drag. The latter force is momentum transfer from ions to particles by either collisions or orbital motion. This force critically depends on the charge on the particle and the shape of the sheath surrounding the particle. In this work, we report on a pseudoparticle-in-cell (PIC) simulation of the transport of electrons and ions in the vicinity of dust particles in low pressure glow discharges. The simulation produces the electrical charge on the dust particle, the sheath structure around the dust particle and the orbital dynamics of the ions. A companion molecular dynamics simulation uses these parameters to produce ion-dust and electron-dust particle cross sections for momentum transfer and collection. Results will be discussed for charge, sheath thickness, cross sections and viscous ion drag forces on dust particles as a function of radius and plasma parameters. >

Self‐consistent theory of ion acoustic waves in a dusty plasma

Abstract: From a self‐consistent theory for dust‐charging, the linear dispersion relation for ion acoustic waves is derived. It is shown that the waves are damped because of energy exchange with the dust‐charging process.

Mathematical model for laser ablation to generate nanoscale and submicrometer-size particles

Abstract: Nanosize particles have several interesting features for synthesizing new materials with improved properties compared to the coarser-grained conventional materials, but the rate of production of such particles in various processes is usually very low. However, laser ablation is expected to give a higher yield. In this paper, a gas-dynamical model is developed for analyzing the plasma formation, and the velocity and particle-size distributions in the plasma during laser ablation. The melting and evaporation rates are determined by using the heat-conduction equation and the Stefan condition. To account for the discontinuity of various state variables across the Knudsen layer, jump conditions are used; the gas-dynamics equations are solved to study the convective flow of the metal particles in the region above the Knudsen layer. The plasma physics is used to model the formation of plasma and to compute the laser-beam attenuation coefficient and ion concentration in the plasma. The particle-size distribution in the plasma is determined by using a droplet-growth theory. Nanosize particles are found to exist near the vaporization front, and the particle size increases as one moves towards the plasma boundary. Similarly, the velocity of the particles is found to be higher at the plasma boundary than that inside the plasma.

Self-consistent charge dynamics and collective modes in a dusty plasma.

Abstract: Charges on dust grains immersed in a plasma may vary as a consequence of plasma currents falling onto their surface. Such charge variations might lead to many collective phenomena in the dusty plasma. We have written a set of multifluid equations that describe the charge dynamics of the dust particles self-consistently, i.e., as a consequence of loss in the electron and ion number densities. Various collective phenomena related with dust- and ion-acoustic modes are then studied in the context of these equations. Consistency of the results with previous work and aspects arising due to the present self-consistent charge dynamics are discussed.

Levitation and dynamics of a collection of dust particles in a fully ionized plasma sheath

Abstract: Examines the dynamics of a collection of charged dust particles in the plasma sheath above a large body in a fully ionized space plasma when the radius of the large body is much larger than the sheath thickness. The dust particles are charged by the plasma, and the forces on the dust particles are assumed to be from the electric field in the sheath and from gravitation only. These forces will often act in opposite directions and may balance, making dust suspension and collection possible. The dust particles are supplied by injection or by electrostatic levitation. The ability of the sheath to collect dust particles, will be optimal for a certain combination of gravitation and plasma and dust particle parameters. In a dense dust sheath, the charges on the dust particles contribute significantly to the total space charge, and collective effects become important. These effects will reduce the magnitude of the sheath electric field strength and the charge on the dust particles. As dust particles are collected, the dust sheath is stretched and the largest dust particles may drop out, because the sheath is no longer able to suspend them. In a tenuous dust sheath, the inner layer, from the surface and about one Debye length thick, will be unstable for dust particle motion, and dust will not collect there. In a dense dust sheath, collective effects will decrease the thickness of this inner dust-free layer, making dust collection closer to the surface possible. By linearization of the force and current equations, the necessary and sufficient conditions for a stable dust sheath are found. The authors consider conditions which resemble those of planetary system bodies, but the results may also be of relevance to some laboratory plasmas. >

Langmuir wave instability in a dusty plasma.

Abstract: By means of a self-consistent theory for dust charging, it is shown that the dust-charge relaxation process leads to an instability of the Langmuir waves.

Spatial distributions of dust particles in plasmas generated by capacitively coupled radiofrequency discharges

Abstract: The transport of particles ('dust') in low-pressure electrical glow discharges is of interest with respect to contamination of semiconductor wafers during plasma etching and deposition. The distribution of dust particles in these reactors is determined by a variety of forces, the most important being electrostatic, viscous ion drag, gravitational, thermophoretic and neutral fluid drag. In this paper we present results from a series of computer models to predict the spatial distribution of dust particles in capacitively coupled electrical glow discharges considering these forces. The results are parametrized over power deposition, gas flow and particle size. We find that the spatial distribution of dust depends on the spatial dependence of the sheaths and plasma potential in bulk plasma which in turn depend upon the electrical topography of the surfaces. Experimentally observed 'dome' and 'ring' distributions of dust particles are computationally reproduced for specific combinations of discharge power particle size and substrate topography.

Measurement of dust grain charging in a laboratory plasma

Abstract: The authors present an experimental system capable of causing dust grains to interact with a plasma in a well-controlled manner and measuring the accumulated charge. Results indicate that the measured charge accumulated on 0.1 /spl mu/m diameter bismuth particles agrees to the first order with simple charging models. With further refinement of the diagnostic system, it is expected that detailed comparisons can be made with existing models. >

Mutual shielding of closely spaced dust particles in low pressure plasmas

Abstract: The transport of particles (‘‘dust’’) in low pressure electrical glow discharges is of interest as a result of their role in contaminating wafers during plasma etching and deposition of semiconductors. Particles (10s nm to many micrometers) negatively charge in glow discharges and, to first order, appear to be massively large negative ions around which sheaths develop. The electrical and fluid forces acting on dust particles in plasma processing discharges may cause the interparticle spacing to be less than the shielding distance around particles. The mutual shielding of dust particles is therefore of interest. In this article, we report on results from a pseudoparticle‐in‐cell simulation of the mutual shielding of two adjacent dust particles. Results will be discussed for charge, potential, and electrostatic forces on dust particles as a function of particle size and separation distance between two particles. We found that two closely spaced particles not only shield each other but can shadow their partner, thereby resulting in asymmetric charging of otherwise identical particles.

On the thermophoretic force close to walls in dusty plasma experiments

Abstract: One observes in radiofrequency-heated vacuum chambers that dust, if present or being produced within the chamber, may float in layers close to both the upper and lower electrodes. Important forces on the dust are the electric force, gravity, plasma drag and the thermophoretic force, which is caused by temperature gradients in the background neutral gas in the vacuum chamber. We here discuss the thermophoretic force and show that the normally adopted formula for this force, which is computed on the assumption of an infinite plasma in all directions from the dust, must be modified when close to the plasma chamber walls. Taking into account the closeness of the plasma walls, we find that the thermophoretic force will be reduced out to many neutral gas molecular collision lengths from the wall, compared with the results from the standard formula. This modification of the thermophoretic force should be of importance for the force equilibrium and stability of dust in the dust layers observed in so many dust levitation experiments.

A study of X-ray and infrared emissions from dusty nonradiative shock waves

Abstract: We have constructed models that predict the dynamic evolution and infrared (IR) emission of grains behind nonradiative shock waves. We present a self-consistent treatment of the effect of grain destruction and heating on the ionization structure and X-ray emission of the postshock gas. Incorporating thermal sputtering, collisional heating, and deceleration of grains in the postshock flow, we predict the IR and X-ray fluxes from the dusty plasma as a function of swept-up column density. Heavy elements such as C, O, Mg, S, Si and Fe are initially depleted from the gas phase but are gradually returned as the grains are destroyed. The injected neutral atoms require some time to 'catch up' with the ionization state of the ambient gas. The nonequilibrium ionization state and gradient in elemental abundances in the postshock flow produces characteristic X-ray signatures that can be related to the age of the shock and amount of grain destruction. We study the effects of preshock density and shock velocity on the X-ray and IR emission from the shock. We show that the effects of graindestruction on the X-ray spectra of shock waves are substantial. In particular, temperatures derived from X-ray spectra of middle-aged remnants are likely to be overestimated by approximately 15% if cosmic abundances are assumed. Due to the long timescales for grain destruction in X-ray gases over a wide range of temperatures, we suggest that future X-ray spectra studies of supernova remnants be based on depleted abundances instead of cosmic abundances. Our model predictions agree reasonably well with IRAS and Einstein IPC observations of the Cygnus Loop.

Plasma particle interactions

Abstract: We present some results from numerical, fluid and particle models of non-thermal low pressure RF discharges contaminated by dust particles. These models have been used (1) to show the effect of the presence of dust particles on the electrical characteristics of an RF discharge, (2) to study the charge and floating potential of dust particles in a low pressure plasma from situations where the particles can be considered isolated to situations where they interact electrostatically and (3) to study the effect of the electrode geometry on the spatial distribution of dust particles in an RF discharge. The results confirm the existence of electrostatic traps close to the plasma-sheath boundary whose shape is very sensitive to the electrode configuration.

A two-dimensional particle-in-cell model of a dusty plasma

Abstract: Dusty plasmas are present in comets, in the ring systems of the outer planets, and in the interstellar medium A two-dimensional particle-in-cell (PIC) model of a dusty plasma is presented in this paper The PIC code is best suited for modeling the plasma-dust interaction for large grains, with diameters of the order of a centimeter We have modeled the charging process for an individual dust grain and the associated potential pattern in the surrounding plasma We have also considered the case of a large number of grains in a plasma, with intergrain separations of the order of the Debye length, and have shown that the plasma becomes depleted and the charge on a dust grain is reduced, as other workers in this field have predicted (cf C K Goertz, 1989) We examine the electron and ion distribution functions in the vicinity of a charged grain and demonstrate that the ions near a grain have clearly been accelerated by the electrostatic potential

Damping and absorption of high‐frequency waves in dusty plasmas

Abstract: By employing standard statistical techniques of linear fluctuation theory, it is shown how the ensemble averages over statistical distributions of charged massive grains in a dusty plasma can result in ‘‘average’’ dispersion relations for high‐frequency electromagnetic and electrostatic waves. The dispersion relations admit solutions for complex frequencies and wave numbers, suggesting that the waves can be damped (Im ω≠0) or absorbed (Im k≠0) in a dusty plasma.

Transport and heating of small particles in high density plasma sources

Abstract: Calculations of the forces experienced by particles in high density plasma sources such as an electron cyclotron resonance system indicate that particles can accelerate in the plasma and travel through sheaths. This is due primarily to the very high ion fluxes in these systems and relatively low sheath potentials. The net result is less particle trapping and growth in the plasma compared to conventional plasma sources. The temperature of the particles is calculated from an energy balance. The particles heat due to electron‐ion recombination on their surface, and they cool by radiation and conduction to the neutral gas. Because of the high plasma density and low neutral pressure, the particles may reach high temperatures (T∼1000 K).

Nucleation, growth, and morphology of dust in plasmas

Abstract: An account is given of the dust particles appearing in weakly ionized plasmas. The dust is found in many discharges, especially in electronegative gas plasmas under conditions of rf excitation similar to those used in plasma processing of integrated circuits. Measurements have been made of the plasma-trapped dust, interrogated by Mie scattering of laser light. It appears that the rapid early growth is controlled by ion accretion across the surface of the relatively large Debye sheath. The microscopic properties of the dust reveal that the particles are often almost monodispersive in size. Under some conditions, the dust morphology has fractal characteristics and considerations of growth conditions point to the critical role of negative ions and ion-assisted deposition 1,Introduction. Dust occurs ubiquitously in many plasmas including astrophysical plasmas, combustion, gas lasers and materials-processing arc discharges. The presence and electrical charging of solid particles in weakly ionized plasmas are important in many technical areas. These areas include aeronomy, thunderstorms, the conductiyity of rocket or engine exhausts, the electrostatic precipitation of particles, and electrostatic effects of condensation in supersonic flows. Grains in space are of special interest because of their importance in forming molecular hydrogen and providing a pathway to more complex molecules. Recent studies (1-5), applying laser probing (Mie scattering) to low pressure rf excited plasma research reactors and to etching and deposition reactors that are used in integrated circuit manufacturing, have shown that particles also often occur in these plasmas, and can adversely affect the manufacturing process and the integrated circuit or material properties. 2. Dust in RF Discharges. The typical parallel plate reactor has electrodes 10-20 cm. diameter separated by 2-10 cm., depending on the gas mixture and wafer process. The etching reactors are usually run at pressures below 0.3 Torr and thin film deposition reactors at a few Torr. The yields of plasma enhanced processing reactors are often degraded by particle formation.The sizes of the cluster or particles that are observed in the reactors range from tens of Angstroms to several microns and, under some circumstances, as large as 10-1 cm. A typical dust particle of one micron size contains 108-109 atoms. The particles can be introduced by several mechanisms. These include flaking from the substrate or walls, clustering and polymerization of the gases or substrate materials, sputtered material, ablation, and aerosols from the gas supply. Particles of micron size will be lifted off a surface in relatively small electric fields. 3. Experimental Observations. (a) nucleation. We found that dust is readily formed and trapped under conditions of rf excitation and low pressure especially when the discharge approaches the situation of two opposing negative glows. The origin of dust particles in laboratory plasmas therefore has been studied in low pressure discharges under conditions of low rf excitation up to 100 kHz. Glass discharge tubes were used with plane parallel disk graphite electrodes and a variety of electrode sizes and electrode spacings.The electrodes were usually mounted vertically so that on switch-off of the discharge the dust particles fell to the bottom of the chamber and not onto the electrodes. Equal area and up to 4 1 area ratio electrodes have been used. To trap the dust, good radial symmetry of the discharge and electrode alignment were imperative. The discharges were operated without gas flow with selected gas mixtures at a known initial pressure. The discharge volume was interrogated for particles using either a cw argon ion her operating at 4880 A, or a krypton ion laser at 6401 A. (Fig. 1) Particles rapidly formed throughout the volume in gases (eg CO) where electron impact-induced dissociation produces carbon atoms, and also in inert gas plasmas with carbon electrodes; the material then must originate from the electrodes, by ion- or metastable-induced sputtering (6). These processes increase the vapor pressure of carbon atoms many times over the equilibrium value for the gas temperature. Qualitatively, under such conditions homogeneous nucleation occurs with the critical size for a stable 'cluster' extending to the dimer. The nucleation to a crystallite at high pressures is enhanced by the very large pressure ratio, equivalent to supercooling, and by the presence of ions (7). Recent studies (8) of nucleation in carbon vapor, in connection with fullerene formation, indicate that the intermediate complex C3* is long-lived which improves the formation probability of larger clusters.