Space- and direction-resolved Langmuir probe diagnostic in rf planar discharges

To consider the anisotropy of the plasma in the sheath regions, Langmuir probe characteristics are measured using a direction-resolving technique. The operation frequency, related to the neutral gas density (/p₀), is chosen in such a way that the electron velocity distribution function (VDF) may be regarded as frozen and the probe diagnostic may be performed without time resolution. In order to prevent convolution of the VDF from the effect of the time-dependent plasma potential, the rf component of the probe bias voltage is compensated to a minimum. The plasma potential, the mean energy of the electrons, and the electron density, averaged over the discharge bulk, are presented as functions of the discharge current and the neutral gas pressure for the O₂ gas. By means of a reactor model based on the theory of plane probes and using the plasma parameter measured, the sheath and bulk portions of the maintaining voltage are separated. In this procedure the thickness of the sheaths in front of the electrodes and the phase difference between discharge current and maintaining voltage are also obtained.     

Attachment and ionization in a self-consistent treatment of the electron kinetics of a collision-dominated rf plasma

This paper deals with the self-consistent determination of the rf field amplitude for sustaining the steady-state collision-dominated weakley ionized plasmas in the bulk of the rf discharge and of the time-resolved behavior of the isotropic part of the distribution function as well as of relevant macroscopic quantities in plasmas whose particle loss is dominantly determined by electron attachment. The strict timeresolved treatment is based on the nonstationary Boltzmann equation of the electrons and its numerical solution including, apart from electron number conservative collision processes, the electron attachment and ionization. The investigations are related to an rf plasma in a model gas and in SF₆ and are performed for reduced rf field frequencies around 10 MHz Torr^–1 which are of particular interest from the point of application of rf discharges for plasma processing. The numerical results show that a large field amplitude of around 160 V cm^–1 Torr^–1 is necessary to maintain the discharge and that the isotropic distribution, the relevant collision frequencies for attachment and ionization, and the electron density undergo a large modulation during a period of the rf field.     

Electron kinetics of collision dominated r.f. bulk plasma in CO: The role of second-kind collisions

Electron energy distribution functions (EEDF) and related properties in the bulk region of the rf CO plasma at the reduced rf field frequency /p₀=×10⁷ sec^–1 torr^–1 have been calculated by solving the time-dependent spatially homogeneous Boltzmann equation in the presence of second-kind collisions and have been interpreted on a microphysical basis. The results show that second-kind collisions (vibrational and electronic) strongly affect the temporal evolution of EEDF, of the mean energy, and of the mean collision frequencies for vibrational and electronic excitation processes, as well as for ionization. In particular, second-kind collisions in the CO rf bulk plasma strongly decrease the modulation of the mean ionization frequency during its periodical alteration in the rf field. Furthermore, the effect of second-kind collisions on an approximate determination of the time-averaged EEDF in the rf bulk plasma using the so-called effective-field appriximation has been estimated.     

Electron kinetics in a collision-dominated SiH4 rf plasma including self-consistent rf field strength calculation

The electron kinetics of a collision-dominated rf plasma in silane has been studied by solving the nonstationary electron Boltzmann equation. Ionization and attachment processes and the spatially averaged electron loss to the plasma wall by ambipolar diffusion have been included in the kinetic approach. This makes it possible to calculate, in addition to the time-resolved energy distribution, the self-consistent rf field amplitude which is necessary for the maintenance of the steady-state rf discharge. The impact of the rf field frequency, of the density ratio of negative ions to electrons, and of superelastic (second kind) collisions with excited silane molecules was studied. In particular, large, rf field amplitudes of about 100 V cm^–1 Torr^–1 result, connected with large modulations of the energy distribution for field frequencies in the megahertz region.     

Anode Magnetron Enhanced DC Glow Discharge Processes for Plasma Surface Cleaning and Polymerization

The objective of this study was to examine some fundamental factors involved in the design and construction of the anode magnetron dc glow discharge processes as well as its performance in plasma cleaning and polymerization. Those advantages of anode magnetron include the capability of the magnetron to operate at low pressure, as well as decreasing the thickness of cathode dark space, i.e., the negative glow which contains a higher concentration of ions and active species was more closely to the cathode surface, which makes the plasma surface cleaning and polymerization an effective and uniform processes. The deposition rate at a given discharge power is increased by the presence of anode magnetrons, and is also much higher relative to rf and af. The refractive index of dc plasma film at a given polymer thickness (such as TMS, 70 nm, RI: 2.4) is higher than rf, af, and cascade arc plasma (RI: 1.6–1.7).     

Optimization of an rf-powered magnetron glow discharge for the trace analysis of glasses and ceramics

A radiofrequency (rf) powered planar magnetron glow discharge ion source has been designed and coupled to a double-focusing mass spectrometer. Superposition of the electrical field of the plasma in the cathode dark space and the magnetic field obtained from a ring-shaped magnet located directly behind the sample (cathode) form the electron traps and enhance the sputtering and ionization efficiency of the ion source. In order to establish optimum conditions for the trace analysis of nonconducting materials, mass spectrometric studies have been carried out on the ion signal intensities and energy distributions of analyte and discharge gas ions depending on pressure.     

Negative ion-containing plasma in parallel-plate radio-frequency discharge in oxygen

The properties of a plasma in a parallel-plate radio frequency (rf) symmetric discharge of 13.56 MHz in oxygen have been investigated. The plasma contains negative ions. The temperature and density of the negative ions have been determined from the slope and height of the semilog-plotted second derivative of the probe characteristics. The laser photodetachment technique is applied, and data indicating the existence of negative ions are obtained. The negative ion densities deduced by both methods are comparable. The negative ion temperature is found to be from 0.1 to 0.6 eV, depending on the pressure and rf power density. The electron energy distribution f(E) deviates from Maxwellian and has a depletion of low-energy parts, which may be due to the loss of low-energy electrons in attachments. A shifted annular-shaped distribution is fitted to the measured f(E), and the data analysis is made according to this form of f(E).     

Optimization of an rf-powered magnetron glow discharge for the trace analysis of glasses and ceramics

A radiofrequency (rf) powered planar magnetron glow discharge ion source has been designed and coupled to a double-focusing mass spectrometer. Superposition of the electrical field of the plasma in the cathode dark space and the magnetic field obtained from a ring-shaped magnet located directly behind the sample (cathode) form the electron traps and enhance the sputtering and ionization efficiency of the ion source. In order to establish optimum conditions for the trace analysis of nonconducting materials, mass spectrometric studies have been carried out on the ion signal intensities and energy distributions of analyte and discharge gas ions depending on pressure.     

Spatially Dependent Kinetics of Electrons and Excited Atoms in the Column Plasma of a He–Xe dc Discharge

The radially varying kinetics of electrons and excited atoms in the cylindrical axially homogeneous positive column of a dc glow discharge in a gas mixture of helium and 2% xenon was studied. The experimental investigations comprise the radially resolved measurements of the isotropic part of the electron velocity distribution function (EVDF) using a single-probe technique and of the densities of atoms in the lower excited states by using a laser diode absorption method. The theoretical investigations are based on the solution of the space-dependent kinetic equation for the EVDF and the balance equations of excited gas atoms. Besides a strict solution, various simplified treatments of the electron kinetics as the conventional homogeneous approach and the nonlocal approach have been applied. The electron kinetic behavior in the helium–xenon column plasma changes remarkably with increasing helium gas pressure from a distinctly nonlocal behavior at a low pressure of 100 Pa to a nearly local behavior at a medium pressure of 600 Pa.     

Electron-impact-induced anisotropic etching of silicon by hydrogen

The etch rate of silicon in a hydrogen low-pressure discharge plasma can be strongly enhanced by electron bombardment, reaching presently up to 1000 Å/min. The etch rate increases linearly with increasing electron current density and hydrogen pressure (range 0.05–0.7 mbar) and decreases with increasing temperature, yielding an activation energy of –4.2 kcal/mole in a temperature range of 80 to 300°C. The etching remains anisotropic within the whole pressure range studied.     

Measurements of the degree of dissociation in oxygen DC discharges: Comparison of the ozone method with the Wrede-Harteck method

The degree of dissociation of oxygen has been measured in the positive column plasma of a glow discharge by two different methods (the ozone method and the Wrede-Harteck method). The results of both methods are in a good agreement. The pressure was varied from 66 to 266 Pa, the discharge current from 4 to 100 mA, and the discharge tube was of 2.0 cm inside diameter. As expected, the degree of dissociation increases with increasing discharge current and decreases with increasing pressure. Calculations of the degree of dissociation show that the main loss process of the oxygen atoms is caused by the recombination on the wall of the discharge tube for pressures up to several 100 Pa. We found that the value of the recombination coefficient for the glass wall of the discharge tube (4.6·10^–4) is about a factor 2 larger than that for a tube outside of the discharge (2.2·10^–4). Ozone processes, including metastable oxygen molecules, have increasing influence on the balance of the oxygen atoms as the pressure increases.     

Model of the Negative DC Corona Plasma: Comparison to the Positive DC Corona Plasma

A numerical model of the negative DC corona plasma along a thin wire in dry air is presented. The electron number density and electric field are determined from solution of the one-dimensional coupled continuity equations of charge carriers and Maxwell's equation. The electron kinetic energy distribution is determined from the spatially homogeneous Boltzmann equation. A parametric study is conducted to examine the effects of linear current density (0.1–100 A per cm of wire length), wire radius (10–1000 m), and air temperature (293–800 K) on the distribution of electrons and the Townsend second ionization coefficient. The results are compared to those previously determined for the positive corona discharge. In the negative corona, energetic electrons are present beyond the ionization boundary and the number of electrons is an order of magnitude greater than in the positive corona. The number of electrons increases with increasing gas temperature. The electron energy distribution does not depend on discharge polarity.     

Influence of Gas Atmosphere in an Ion Source of a Laser Mass Spectrometer on the Results of Elemental Analysis

The effect of bleeding oxygen or a mixture of inert gases (Ne, Ar, and Xe) in the ion source of an EMAL-2 mass spectrometer on the results of elemental analysis (EI 696 steel, L 68 brass, and Br 663 bronze standard reference samples) was examined. Filling the ion source with gases to a pressure of 10^–2 Pa increased the number of singly charged ions and decreased the number of multiply charged ions in a laser-induced plasma. This fact allowed us to enhance analytical signals by a factor of 1.5. The effect was most pronounced for ions of elements whose second ionization potentials lie in a range of 15–20 eV. The effects were almost identical for bleeding oxygen and inert gases.     

Experimental and Theoretical Studies of a Pulsed Microwave Excited Ar/CF4 Plasma

The present work deals with a pulsed microwave discharge in an Ar/CF ₄ gas mixture under a low pressure (1–10 mbar). The discharge chamber developed has a cylindrical geometry with a coupling window alternatively made of quartz or alumina. The setup allows one to investigate the plasma–wall interactions (here etching of the quartz window) and the ignition process of the pulsed microwave plasma. Microwave pulses with a duration of 50–200 s and repetition rate between 1 and 10 kHz are typical for the experiments. The space-time behavior of the fluorine number density in the discharge has been investigated experimentally by optical actinometry. The discharge kinetics is modeled using electron-transport parameters and rate coefficients derived from solutions of the Boltzmann equation. Together with the solution of the continuity and electron balance equations and the rate equations describing the production of CF _x (x=2, 3, 4) radicals and F atoms, a good agreement between experimental and theoretical data can be achieved.     

Optical and Electrical Properties of Polymerizing Plasmas and Their Correlation with DLC Film Properties

Diamond-like carbon (DLC) films were grown from radiofrequency plasmas of acetylene-argon mixtures, at different excitation powers, P. The effects of this parameter on the plasma potential, electron density, electron temperature, and plasma activity were investigated using a Langmuir probe. The mean electron temperature increased from about 0.5 to about 7.0 eV while the mean electron density decreased from about 1.2 × 10⁹ to about 0.2 × 10⁹ cm^–3 as P was increased from 25 to 150 W. Both the plasma potential and the plasma activity were found to increase with increasing P. Through actinometric optical emission spectrometry, the relative concentrations of CH, [CH], and H, [H], in the discharge were mapped as a function of the applied power. A rise in [H] and a fall in [CH] with increasing P were observed and are discussed in relation to the plasma characteristics and the subimplantation model. The optical properties of the films were calculated from ultraviolet-visible spectroscopic data; the surface resistivity was measured by the two-point probe method. The optical gap, E _G, and the surface resistivity, _s, fall with increasing P. E _G and _s are in the ranges of about 2.0–1.3 eV and 10¹⁴–10¹⁶ /, respectively. The plasma power also influences the film self-bias, V _b, via a linear dependence, and the effect of V _b on ion bombardment during growth is addressed together with variation in the relative densities of sp² and sp³ bonds in the films as determined by Raman spectroscopy.     

Atomic Oxygen Concentration in a Flowing Post-Discharge Reactor

Concentration of neutral oxygen atoms in the flowing post-discharge of a pure oxygen microwave discharge at different experimental conditions was determined with a nickel catalytic probe. The post-discharge reactor was setup for metal surface cleaning. It worked at the pressure between 20 and 100 Pa and at output power of the microwave plasma generator between 80 and 150 W. At those experimental conditions the O-atom density was found to be of the order of 10²¹ m^–3. It increased both with increasing pressure and microwave power. The degree of dissociation of oxygen molecules, on the other hand, decreased with increasing pressure.     

Kinetics of Gaseous Product Formation in the Surface Treatment of Polypropylene with Nitrogen–Oxygen Plasmas

The results of measurements on the composition and the formation rates of gaseous products in the surface treatment action of polypropylene with a low-temperature dc discharge plasma in a nitrogen–oxygen mixture are reported. The amount of oxygen in the mixture was varied within 0–100%. It was found that almost stoichiometric plasma-induced oxidative degradation occurred in an oxygen plasma, whereas the buildup of oxygen-containing functional groups on the surface took place in a mixed nitrogen–oxygen plasma at a nitrogen content of 10–60%.     

The Mechanisms of Formation and Decay of Atoms in N2-H2 Plasmas

The formation rate and number densities of H(² S) and N(⁴ S) atoms, rate constants of their heterogeneous decay, and some electrophysical characteristics of plasma were measured in the positive column of a dc discharge in an H₂–N₂ gas mixture at a pressure of 266 Pa and a discharge current of 50 mA. From these data, by the combined solution of the Boltzmann equation, vibrational kinetics equations for ₂(X ¹_g) and N₂(X ¹_g), chemical kinetics equations, and plasma energy and heat balance equations, it was found that vibrationally excited nitrogen molecules must play a substantial role in the formation of both hydrogen and nitrogen atoms.     

Thermal effects in low-pressure plane plasma apparatus for thin films applications

Thermal effects in a low-pressure plane plasma discharge were obtained in a novel implementation of triode sputtering method. This plane plasma discharge is formed in a relatively low vapor pressure of 0.03-0.65 Pa. Electron beam temperature and ion beam concentration distribution, as well as their dependence on argon pressure within the plasma, were experimentally studied, using the Langmuir probe technique. The influence of an external magnetic field on the ion beam concentration, and electron beam temperature, were studied too. As a result of these studies, sputtering of various materials was done using the novel plane plasma discharge method. This method enables the deposition of homogeneous thin film coatings. Analysis is done on Cu sputtered layers with plane plasma discharge. This revised version was published online in July 2006 with corrections to the Cover Date.