Showing papers in "Plasma Sources Science and Technology in 1994"
TL;DR: Several models that predict the charge of particles in a plasma are reviewed in this paper, including orbit-limited probe theory, which can be improved by adding several effects: charge reduction at high dust densities, electron emission, ion trapping and fluctuations.
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.
336 citations
TL;DR: In this paper, a Langmuir probe for the study of high-density RF discharges has been developed and tested in a helicon discharge in which the RF potential (approximately=100 V) is much larger than the electron temperature (approximately = 4 eV).
Abstract: A Langmuir probe for the study of high-density RF discharges has been developed and tested in a helicon discharge in which the RF potential ( approximately=100 V) is much larger than the electron temperature ( approximately=4 eV). Carbon probe tips are used to minimize erosion by ion sputtering. Miniature RF chokes located close to the probe tip present RF impedances of 150 k Omega at the operating frequency of 27.12 MHz and 300 k Omega at the first harmonic at 54.24 MHz. It is further necessary to drive the probe tip to follow the RF fluctuations by coupling it to a larger floating electrode. We have been able to measure Te values as low as 3.4 eV in argon plasmas of 1013 cm-3 density; these temperatures are 1.6 eV lower than ones measured by probes with chokes alone, and 2.3 eV lower than measured by uncompensated probes.
296 citations
TL;DR: In this paper, the growth of particle size has been measured in a low-pressure argon-silane plasma using high-resolution transmission electronic microscopy and the results showed that formation and growth of dust particles is an homogeneous process; the first generation size distribution is monodispersed; and the growth kinetics reveals a three-step process from molecular ions to large particles.
Abstract: The growth of particle size has been measured in a low-pressure argon-silane plasma using high-resolution transmission electronic microscopy. The results show that formation and growth of dust particles is an homogeneous process; the first generation size distribution is monodispersed; and the growth kinetics reveals a three-step process from molecular ions to large particles. Together with measurements of particle concentration obtained by laser light scattering, these measurements give a clear indication that the growth proceeds through three successive steps: (i) 'rapid' formation of crystalline clusters (as shown by dark-field high-resolution transmission electron microscopy) with concentrations of up to 1010 cm-3; (ii) formation of aggregates, of diameters up to 50 nm, by coagulation (during coagulation the particle concentration decreases dramatically); and (iii) growth of the particles with a constant concentration by surface deposition of SiHx radicals, whilst the numerical density remains constant. Laser-induced particle explosive evaporation has been performed using a XeCl (308 nm) laser. This experiment allowed detection of nanocrystallites and also the beginning of their coagulation and gave clear evidence of the temperature effect on particle formation.
268 citations
TL;DR: In this paper, the authors report theoretical and experimental investigations of an atmospheric pressure, microwave plasma torch with axial gas injection (torche a injection axiale design), which is a waveguide-based structure comprising both waveguide and coaxial elements serving the purpose of wavemode conversion and impedance-matching.
Abstract: We report theoretical and experimental investigations of an atmospheric pressure, microwave plasma torch with axial gas injection (torche a injection axiale design). It is a waveguide-based structure comprising both waveguide and coaxial elements serving the purpose of wavemode conversion and impedance-matching. The device includes features common to various waveguide-fed torches disclosed previously by other authors, but not yet modelled. Our paper provides a simple equivalent circuit description of the torch operation that accounts for its impedance-matching, power transfer to plasma and tuning characteristics, as verified experimentally. From the outcome of the model and using our experimental results, we introduce new features in the torch design that enable one to optimize its performance. We also examine ways of simplifying its structure and operation.
198 citations
TL;DR: In this article, the external electrical characteristics of inductively coupled argon RF discharges at 13.56 MHz have been measured over a wide range of power at gas pressures ranging from 3 mTorr to 3 Torr.
Abstract: The external electrical characteristics of inductively coupled argon RF discharges at 13.56 MHz have been measured over a wide range of power at gas pressures ranging from 3 mTorr to 3 Torr. External parameters, such as coil voltage, current and phase shift, were measured. From these measurements the equivalent discharge resistance and reactance, the power transfer efficiency and the coupling coefficient between the primary coil and the plasma were determined as a function of discharge power and gas pressure. The efficient RF power transfer and the large value of the effective electron collision frequency found here at low gas pressure suggest some collisionless electron heating mechanisms. This mechanism is identified as non-local electron heating in the inhomogeneous RF field due to spatial dispersion of the plasma conductivity.
131 citations
TL;DR: An annular waveguide resonator with axial slots on its inner side acts as a field applicator to sustain a plasma at 2.45 GHz in a fused silica cylinder of 16 cm in diameter and 49 cm in length.
Abstract: A new large-volume microwave plasma source of interest for technological applications has been developed. An annular waveguide resonator with axial slots on its inner side acts as a field applicator to sustain a plasma at 2.45 GHz in a fused silica cylinder of 16 cm in diameter and 49 cm in length. The distance between slots is equal to a waveguide wave length. The slot antenna (SLAN) extends axially for about 9 cm. The plasma fills the tube axially as a result of surface wave propagation at the plasma-fused silica interface. The geometrical properties of the source were optimized by use of a numerical code allowing calculation of electric and magnetic fields inside the slot antenna. The calculated field patterns fit perfectly to those collected by use of a small loop antenna inside the SLAN without a plasma. A long time stable operation of the plasma source for pressure range from 0.01 to 1 mbar and power range from 50 to 1200 W was achieved. Argon plasma was characterized by use of a double Langmuir probe. Typical results are an ion density of 1.2*1012 cm-3 and an electron temperature of 1.2 eV*11604 K eV-1 for a microwave power of 1.2 kW and a gas pressure of 1 mbar. The worst case azimuthal variation of the ion density for high pressure (1 mbar) does not exceed 25%. For low pressure (0.01 mbar) the azimuthal homogeneity is better than 5%.
119 citations
TL;DR: In this paper, the authors examined the initial mechanism by which silicon clusters start growing up to the point where they suddenly aggregate into multiply charged particles and modify the discharge regime, and provided a review of basic data concerning anions in SiH4 plasmas and analyzed mass spectrometric data showing that anion-molecule reactions at Langevin rates probably dominate the cluster growth kinetics below 100 Si atoms whereas anion neutral and neutral neutral condensation at size-scaling collision rates govern the subsequent homogenous nucleation regime.
Abstract: After having distinguished five successive steps in the temporal evolution of a powder-forming SiH4 radiofrequency glow discharge, we examine the initial mechanism by which silicon clusters start growing up to the point where they suddenly aggregate into multiply charged particles and modify the discharge regime. This 'induction' period can be much longer than the diffusion time of positive ions and neutral radicals, which implies that cluster growth involves negative ions (anions). We provide a review of basic data concerning anions in SiH4 plasmas and analyse mass spectrometric data showing that anion-molecule reactions SinH2n+1-+SiH4 to Sin+1H2n+3-+H2 at relatively low rate (about 10-12 cm3 s-1), and fast exothermic anion-radical reactions SinHm-+SiHm' to Sin+1Hm+m'-2q-+qH2 at Langevin rates (about 10-9 cm3 s-1), initiate clustering. The effective anion lifetime involves a succession of dissociative attachment to SiH4, detachment or recombination, and attachment to neutral radicals or clusters competing with diffusion out of the plasma. Anion-molecule and anion-radical cluster reactions at Langevin rates probably dominate the cluster growth kinetics below 100 Si atoms whereas anion-neutral and neutral-neutral condensation at size-scaling collision rates govern the subsequent homogenous nucleation regime. At the end of the nucleation period (up to 104 Si atoms) the fraction of singly charged clusters can reach 50%. The reduction of powder formation upon gas heating is attributed to a decrease of the rate of non-dissociative attachment to radicals and neutral clusters.
117 citations
TL;DR: A review of the phenomena associated with particles in low pressure plasmas is presented in this paper, where the source material and its required generation rate, nucleation, charging, growth mechanisms, growth rates, and saturation mechanisms are discussed.
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.
107 citations
TL;DR: In this paper, particle genesis and growth in RF silane plasmas by ion mass spectrometry and light scattering was detected by ion-mass spectrometer and light-scattering.
Abstract: Diagnostics of particle genesis and growth in RF silane plasmas by ion mass spectrometry and light scattering
107 citations
TL;DR: The radiofrequency power coupling efficiency of a planar induction source is determined by measurement of the power dissipated in the matching network and inductive coupler as mentioned in this paper, and the efficiency is highest when the plasma-to-inductor distance is least since the increased mutual inductance reduces the radiofrequency inductor current and, hence, ohmic losses.
Abstract: The radiofrequency power coupling efficiency of a planar induction source is determined by measurement of the power dissipated in the matching network and inductive coupler. Typically one finds the radiofrequency current to vary between 20 and 60 A (root mean square). The equivalent resistance of the coupling network is determined to be 0.09 Omega , and power coupling efficiency to the plasma is therefore found to be 70-90% with the remaining power lost to ohmic heating of the circuit elements. The efficiency is highest when the plasma-to-inductor distance is least since the increased mutual inductance reduces the radiofrequency inductor current and, hence, ohmic losses. Efficiency of power coupling is nearly constant as a function of radiofrequency power between 200 and 2000 W. Somewhat larger currents are required to sustain a discharge at lower pressures (about 1 mTorr) or when the plasma is not magnetically confined, giving rise to lower coupling efficiencies.
99 citations
TL;DR: In this article, the Coulomb liquid model of dust-forming plasmas is used to model the free electron population induced by the presence of the particles, with significant effects on the chemical equilibrium of the plasma.
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.
TL;DR: In this paper, a self-consistent description for the entire arc cathode attachment region, a realistic one-dimensional sheath model has been used, supplemented by an integral energy balance of the ionization zone between the sheath and the arc, and by a differential energy imbalance of the cathode.
Abstract: A theoretical model has been formulated describing the influence of the arc condition and the cathode material and geometry on arc cathode erosion. To arrive at a self-consistent description for the entire arc cathode attachment region, a realistic one-dimensional sheath model has been used. This sheath model is supplemented by an integral energy balance of the ionization zone between the sheath and the arc, and by a differential energy balance of the cathode. For the case of a tungsten cathode in an argon arc, it has been shown that the ion current density is almost 50% of the total current density at low arc currents, while it decreases to about 18% of the total current density and the thermionic electron current density increases to about 82% of the total current density at high currents. It has also been found that heat conduction within the cathode and radiation from the cathode surface control energy transport from the cathode spot at low currents, and that dissipation by thermionic electron release dominates at high currents.
TL;DR: In this paper, a kinetic theory for the electron probe current is presented, which allows us to infer the electron energy distribution function from the measured probe current and its derivatives over a wide range of pressures and magnetic fields.
Abstract: A kinetic theory for the electron probe current is presented. The results allow us to infer the electron energy distribution function from the measured electron probe current and its derivatives over a wide range of pressures and magnetic fields. To recover the electron energy distribution function from probe characteristics two numerical methods are proposed. Probe measurements in a rare gas afterglow plasma were carried out over a wide pressure and energy range. The experimental results are in good agreement with the theory developed in the paper.
TL;DR: In this article, the effects of frequency variation in a high-frequency (13.6-54.4 MHz), low-pressure (250 mTorr), capacitively coupled argon discharge have been investigated.
Abstract: The effects of frequency variation in a high-frequency (13.6-54.4 MHz), low-pressure (250 mTorr), capacitively coupled argon discharge have been investigated. Spatially resolved optical emission and RF voltage and current measurements are reported. Results of fluid model simulations are also presented to aid in the interpretation of the experiments. Scaling of plasma parameters is studied under three conditions: constant frequency, constant applied voltage and constant current. The scaling laws derived here suggest ways to optimize a very high-frequency plasma reactor for thin film etching and deposition applications.
TL;DR: In this paper, the behavior of heavy particles in an argon plasma expanding from a cascaded arc is characterized using high-accuracy spatially resolved Thomson-Rayleigh measurements, and it is shown that the expansion of the heavy particles, neutral particles and ions, is close to adiabatic and that threeparticle recombination has a small effect on the ion density.
Abstract: This paper concerns the behaviour of heavy particles in an argon plasma expanding from a cascaded arc. The plasma is characterized using high-accuracy spatially resolved Thomson-Rayleigh measurements. It is shown that the expansion of the heavy particles, neutral particles and ions, is close to adiabatic and that three-particle recombination has a small effect on the ion density. The measurements are compared with an adiabatic model and are found to be in good agreement. The behaviour of the neutral particles in the shock front is independent from that of the ions. The neutral particle shock front is identical to the shock front found in neutral gases and is accordingly characterized by the neutral particle Mach number. It is experimentally confirmed that the shape of the shock front is of the Mott-Smith type. The motion of the ions in the shock front is influenced by the presence of the electrons in such a way that the electrons are compressed by the ions. The shape of the ion shock front is of the Mott-Smith type, however, with a different Mach number which includes the electron temperature.
TL;DR: In this article, the absolute fluorine atom concentrations in electron cyclotron resonance (ECR) and reactive ion etching (RIE) plasmas were measured by optimizing the actinometry technique.
Abstract: We have measured the absolute fluorine atom concentrations in electron cyclotron resonance (ECR) and reactive ion etching (RIE) plasmas by optimizing the actinometry technique. The major difference between this work and conventional actinometry is that the Ar concentration measurements were performed by a residual gas analyser (RGA). The emission intensities of F (7037 AA) and Ar (7504 AA) were simultaneously measured by an optical multichannel analyser (OMA) and the Ar concentration by a RGA. The F atom concentration at the wafer stage in the CF4 ECR plasma was measured to be (0.4 to 4)*1012 cm-3, the microwave power from 500 to 900 W, pressure from 0.5 (3.7 sccm) to 3.5 mTorr (58 sccm), and a fixed RF bias voltage of -50 V. The F atom concentration of the CF4 ECR plasma was four times larger in the source region than in the downstream region. The F atom concentration of the CF4 RIE plasma for pressures from 13 to 82 mTorr was measured as (0.8 to 4.2)*1013 cm-3, for a CF4 flow rate of 20 sccm, and power inputs from 250 to 1500 W. The F atom concentration was larger in the RIE etcher than in the ECR etcher, but the F atom production efficiency was eight times larger in the ECR etcher than in the RIE etcher for the same power level. In spite of the possibility of a factor two discrepancy in the measurements from absolute values because of the uncertainty in the absolute values of the cross sections, this technique provides a relatively simple and consistent reproducible measurement of F atom concentration to compare operation of the two types of etchers, which is reproducible to +or-10%. We further explored the actinometry for vacuum ultraviolet (VUV) emission region by using F (955 AA) and Ar (1048 AA). A similar trend of F atom concentration was found as for the visible actinometry, but the absolute value of fluorine atom concentration was typically 15% larger for the VUV actinometry.
TL;DR: In this article, a two-dimensional axisymmetric model of a bounded, partially ionized, magnetized glow discharge plasma has been developed, which treats positive ions as particles and electrons as a fluid in a hybrid configuration.
Abstract: A two-dimensional, axisymmetric model of a bounded, partially ionized, magnetized glow discharge plasma has been developed. The model treats positive ions as particles and electrons as a fluid in a hybrid configuration. The results reported are directed towards simulating an electron cyclotron resonance (ECR), microwave-sustained plasma. Microwave power profiles in the plasma are assumed, and the resulting electron and ion transport in the applied magnetic and self-consistent electrostatic field is calculated. Sheaths under typical operating conditions are very thin and the authors apply an analytic sheath model to avoid integration in the sheaths. Typical results are presented for a common ECR reactor geometry and conditions, and comparisons with experimental data are made when possible.
TL;DR: In this paper, a Langmuir double-probe system was developed and tested for measurement in industrial environments, where quick and easy-to-handle methods are required, where data acquisition and calculation of plasma parameters are controlled by a personal computer.
Abstract: We developed and tested a Langmuir double-probe system suitable for measurement in industrial environments, where quick and easy-to-handle methods are required. Data acquisition and calculation of plasma parameters are controlled by a personal computer. An evaluation algorithm is presented and its properties are briefly discussed. Successful operation of the system can be proved in a variety of different plasma configurations, namely in radiofrequency driven, pulsed direct current and microwave plasmas. Typical measurements and the conclusions that have been drawn are presented. Plasma densities were in the range 108-1013 cm-3. From the end user's point of view the main application areas are trouble-shooting, on-line process control and plasma source characterization during the development stages of a plasma plant.
TL;DR: A negative ion plasma is produced by introducing a small amount of SF6 gas into a low-temperature (approximately=0.2 eV) potassium plasma produced in a Q machine.
Abstract: A negative ion plasma is produced by introducing a small amount of SF6 gas into a low-temperature ( approximately=0.2 eV) potassium plasma produced in a Q machine. The density ratio of negative to positive ions is continuously varied in the range up to more than 0.9999, where there appears a remarkable decrease in electron shielding for potential variations, yielding a clear effect on plasma collective phenomena. By introducing fullerene (C60)particles into the Q-machine plasma, we can produce a plasma including large negative C60 ions. This ultrafine particle plasma might prove very attractive in fields of materials science.
TL;DR: In this paper, laser light scattering has been used to monitor the behavior of particles for a wide range of plasma tools, including sputter, etch and deposition tools of planar diode and magnetron-enhanced designs, some results have also been obtained in electron cyclotron resonance tools and radiofrequency inductive tools.
Abstract: Particles or 'dust' in etching or deposition plasmas are an important cause of product yield loss and equipment down-time. Traditional methods of particle control are only partially effective in plasma processing. This is because formation and transport of particles are strongly influenced by plasma electrical and chemical properties. Particle control in plasma processing requires understanding of these effects and their relation to aspects of tool and process design. Laser light scattering has been used to monitor the behaviour of particles for a wide range of plasma tools. This method provides information on location and transport of particles. In some variations, light scattering may also be used for particle size determination. Results have been obtained in sputter, etch and deposition tools of planar diode and magnetron-enhanced designs, Some results have also been obtained in electron cyclotron resonance tools and radiofrequency inductive tools. From this database, differences and common elements are observed for the behaviour of particles. The particle trapping phenomenon is often observed. Particle traps have an important bearing on wafer contamination. Traps cause particles to accumulate into localized regions during plasma operation, only to be suddenly released at the end of the process, thereby contaminating the wafer. Spatially resolved optical emission may be used to map the location and intensity of particle traps. This method also provides a semi-quantitative comparison with two-dimensional modelling studies. It may also be used to optimize grooved electrode design for particle contamination control.
TL;DR: In this paper, the dispersion relation for helicon waves in a cold plasma of radially varying density has been reduced to compact form, and the radial eigenmodes have been computed for different density profiles.
Abstract: The dispersion relation for helicon waves in a cold plasma of radially varying density has been reduced to compact form, and the radial eigenmodes have been computed for different density profiles The results show a marked asymmetry between the left- and right-hand circularly polarized modes: the m=-1 (left) mode has a centrally peaked wave intensity and resonates with a much higher central density than the m=+1 mode Positive feedback is therefore possible, leading to nonlinear channelling of the discharge At a radius where the density falls to a certain value, a singularity arises in the coefficients of the wave equation; care must be taken in integrating through this point This singularity has no physical significance The marked difference between the m=+1 and -1 modes in a non-uniform plasma is caused by a difference in sign of the electron drift along the density gradient Energy deposition is peaked near the radius of the peak in Bz, so that broad, uniform density profiles can be obtained by using the m=+1 mode and narrow, dense columns by using the m=-1 mode These results explain many features observed by various groups over the past two years
TL;DR: In this article, the authors present results from a series of computer models to predict the spatial distribution of dust particles in capacitively coupled electrical glow discharges considering electrostatic, viscous ion drag, gravitational, thermophoretic and neutral fluid drag.
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.
TL;DR: In this article, a model of an electron cyclotron resonance discharge is applied to prediction of the existence and nature of particle trapping, and the model predicts that particles may be heated to temperatures of two to three times room temperature.
Abstract: Once particles are formed or injected into plasmas used for materials processing, such as in plasma etching, plasma-assisted chemical vapour deposition or sputtering plasma systems, the nature of particle transport will largely determine whether a processing surface will be contaminated. We investigate the situation in which the particle density is low enough to ignore particle-particle and particle-plasma interactions. Emphasis is placed on obtaining expressions for the forces experienced by particles. These expressions depend on the local plasma condition: plasma density, electron temperature, positive ion directed and random kinetic energies, electric field and ion mass. We apply a model of an electron cyclotron resonance discharge to prediction of the existence and nature of particle trapping. Model predictions indicate that a high-density source such as an electron cyclotron resonance discharge is unlikely to trap particles mainly because of the large ion drag force sweeping particles out of the discharge. Finally, we present a model of particle heating in discharges. Under typical radiofrequency discharge conditions, particles are generally predicted to be near the neutral gas temperature in the discharge. We have conducted experiments and found results in agreement with these predictions. However, under conditions typically encountered in high-density plasma sources such as an electron cyclotron resonance source, the model predicts that particles may be heated to temperatures of two to three times room temperature.
TL;DR: In this paper, the heating of the electron gas in the shock front of an expanding plasma jet is modelled by means of a quasi-one-dimensional model in combination with a two-dimensional calculation of the electrical properties.
Abstract: The heating of the electron gas in the shock front of an expanding plasma jet is modelled. In the expansion, due to the large pressure gradient, a current is generated which, besides the heating by three-particle recombination, takes care of additional heating of the electron gas. It is shown, by means of a quasi one-dimensional model in combination with a two-dimensional calculation of the electrical properties, that the preheating of the electron gas in front of the shock can be explained by Ohmic dissipation of the generated current density in combination with the electron heat conduction. The outcome of the model is compared with experimental results.
TL;DR: In this article, the authors discussed the thermophoretic force and showed 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.
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.
TL;DR: In this article, the effects of plasma processing conditions on the microstructural properties of silicon powders are presented, which reveal an increase in hydrogen content and a reduction in volume/surface ratio as the modulation frequency of RF power increases.
Abstract: The effects of plasma processing conditions on the microstructural properties of silicon powders are presented. Hydrogenated nanophase silicon powders were prepared using low-pressure and low-temperature square wave modulated RF plasma (13.56 MHz) using pure silane gas. Plasma parameters such as pressure, RF power, plasma modulation frequency, and gas flow rate were varied. In situ analysis by quadrupolar mass spectroscopy and ex situ analysis of the silicon powders by Fourier transform infrared spectroscopy (FTIR) and thermal desorption spectrometry of hydrogen were performed. The thermal desorption spectrometry results show the fundamental differences between the concentrations of hydrogen weakly and strongly bonded in silicon powders as compared to amorphous silicon films. The FTIR analysis also determined the microstructural characteristics of powders and hence their volume/surface ratio. This parameter was determined from the balance of Pj probabilities of having one of the Hj-Si-Si4-j bond arrangements in the powder particles. These results reveal an increase in hydrogen content and a reduction in volume/surface ratio as the modulation frequency of RF power increases. In consequence, higher compactness of silicon powders is associated with long particle residence times inside the plasma as a result of ion bombardment. TEM analysis indicated a considerable dispersion of particle size and some degree of structure of the silicon powder characterized by intergrain linkage. We point out the dominant presence of hydrogen on the particle surfaces (external voids), which may cause the high reactivity of grains, increasing the degree of intergrain linkage.
TL;DR: In this paper, a secondary electron emission-capacitive probe has been used to explore the dynamic behaviour of the sheath at high voltages in a glow discharge plasma generated by inductively coupled RF power (approximately 300 W at 12-13 MHz).
Abstract: A secondary electron emission-capacitive probe has been used to explore the dynamic behaviour of the sheath at high voltages in a glow discharge plasma (ni approximately 1010 cm-3 N2+, T3 approximately 8 eV) generated by inductively coupled RF power ( approximately 300 W at 12-13 MHz). When a high negative potential is applied to the cathode the sheath expands rapidly but comes to a steady-state position within a few microseconds. The potential distribution then remains stable throughout the high-voltage pulse as long as the ionization rate in the plasma outside the sheath region is sufficient to replace the ions lost to the cathode. Around a sphere, this equilibrium can be described by conventional theories of the Child-Langmuir sheath. Previous experimental and theoretical investigations of the evolution of the cathodic sheath are reviewed in the light of these results which are significantly different from recently reported measurements in hot-filament and microwave multidipole discharges. It is concluded that the RF 'self-bias' of the target (which exists before application of the high voltage) and the high ionization rate in the surrounding plasma influence the time-scale on which the sheath comes to equilibrium.
TL;DR: The characteristics of plasma parameters and their spatial structures in a parallel-plates RF discharge in argon modified by the injection of small carbon particles have been investigated in this paper, where a larger peak-to-peak fluctuation of space potential in the bulk plasma region was observed by an emissive probe measurement together with a decrease in the self-bias voltage.
Abstract: The characteristics of plasma parameters and their spatial structures in a parallel-plates RF discharge in argon modified by the injection of small carbon particles have been investigated. A larger peak-to-peak fluctuation of space potential in the bulk plasma region was observed by an emissive probe measurement together with a decrease in the self-bias voltage. The electron energy distribution function in the plasma bulk as measured by an energy analyser also showed the presence of a higher energy tail. The enhancement of an argon atomic line emission in the midgap was consistent with the above results, showing the transition of the discharge mode from the gamma to alpha regimes. Spatial and temporal behaviours of particle density and size were measured by a newly proposed ellipsometric detection method of the laser Mie scattered light. Slow shift of the peak position of density and size distributions towards the sheath edge of the powered electrode was observed.
TL;DR: In this article, the influence of excited atoms on the characteristics of a surface wave produced argon discharge is investigated and a self-consistent model is used to determine simultaneously the electron energy distribution function, the population densities of the 4s states of argon and the maintaining electric field strength.
Abstract: The influence of excited atoms on the characteristics of a surface wave produced argon discharge is investigated. A self-consistent model is used to determine simultaneously the electron energy distribution function, the population densities of the 4s states of argon and the maintaining electric field strength. The theoretical results are compared with spectroscopic measurements of the population densities and with Langmuir probe results for the electron distribution function.
TL;DR: In this article, numerical, fluid and particle models of non-thermal low pressure RF discharges contaminated by dust particles are used to show the effect of the presence of dust particles on the electrical characteristics of an RF discharge, and study the charge and floating potential of particles in a low pressure plasma from situations where the particles can be considered isolated to situations where they interact electrostatically.
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.