Measured Electron Energy Distribution Function Research Articles

Increasing the Measurement Range of Plasma Electron Parameters in the Single Langmuir Probe Method

It is proposed to use a new signal type fed to the single Langmuir probe to measure current—voltage characteristics. It is shown that such an approach makes it possible to increase the range of measured electron energy distribution functions.

Using Poisson-regularized inversion of Bremsstrahlung emission to extract full electron energy distribution functions from x-ray pulse-height detector data

We measured Electron Energy Distribution Functions (EEDFs) from below 200 eV to over 8 keV and spanning five orders-of-magnitude in intensity, produced in a low-power, RF-heated, tandem mirror discharge in the PFRC-II apparatus. The EEDF was obtained from the x-ray energy distribution function (XEDF) using a novel Poisson-regularized spectrum inversion algorithm applied to pulse-height spectra that included both Bremsstrahlung and line emissions. The XEDF was measured using a specially calibrated Amptek Silicon Drift Detector (SDD) pulse-height system with 125 eV FWHM at 5.9 keV. The algorithm is found to out-perform current leading x-ray inversion algorithms when the error due to counting statistics is high.

Comparative analyses of plasma probe diagnostics techniques

The subject of this paper is a comparative analysis of the plasma parameters inferred from the classical Langmuir probe procedure, from different theories of the ion current to the probe, and from measured electron energy distribution function (EEDF) obtained by double differentiation of the probe characteristic. We concluded that the plasma parameters inferred from the classical Langmuir procedure can be subjected to significant inaccuracy due to the non-Maxwellian EEDF, uncertainty of locating the plasma potential, and the arbitrariness of the ion current approximation. The plasma densities derived from the ion part of the probe characteristics diverge by as much as an order of magnitude from the density calculated according to Langmuir procedure or calculated as corresponding integral of the measured EEDF. The electron temperature extracted from the ion part is always subjected to uncertainty. Such inaccuracy is attributed to modification of the EEDF for fast electrons due to inelastic electron collisions, and to deficiencies in the existing ion current theories; i.e., unrealistic assumptions about Maxwellian EEDFs, underestimation of the ion collisions and the ion ambipolar drift, and discounting deformation of the one-dimensional structure of the region perturbed by the probe. We concluded that EEDF measurement is the single reliable probe diagnostics for the basic research and industrial applications of highly non-equilibrium gas discharge plasmas. Examples of EEDF measurements point up importance of examining the probe current derivatives in real time and reiterate significance of the equipment technical characteristics, such as high energy resolution and wide dynamic range.

Plasma diagnostic method using intermodulation frequencies in a Langmuir probe

A plasma diagnostic method using intermodulation frequencies is developed. When dual-frequency (ω1,ω2) voltage signals are applied to a probe, the intermodulation frequencies (ω2±ω1, ω2±2ω1) between the signals are generated due to the nonlinearity of the sheath. From the analysis of the intermodulation frequencies, the plasma parameters, such as the electron temperature and the plasma density, can be obtained. The measured plasma parameters from this method are compared to the results from the measured electron energy distribution function, and they are in good agreement. Because the intermodulation currents originated from the plasma not from the stray component of the measurement system, an accurate measurement of the plasma parameters is achievable.

Investigation of capacitively coupled argon plasma driven at various frequencies and validation of surface waves excitation

The influence of excitation frequency (13.56–96 MHz) on the characteristics of capacitively coupled argon plasma is investigated by means of Langmuir probe and a high-voltage probe. Measurements are performed in argon pressure of 40 and 60 mTorr at a fixed discharge voltage Vpp=200 V. The measured electron energy distribution function EEPFs are a bi-Maxwellian type irrespective of the driving frequency and gas pressure. The electron density and temperatures show peak over frequency range of 54–72 MHz, beyond which it decreases. The non-monotonic dependences of plasma parameters with driving frequency were interpreted in terms of excited surface wave excited at the powered electrode.

Acetylene–argon plasmas measured at a biased substrate electrode for diamond-like carbon deposition: I. Mass spectrometry** This paper was presented as an invited talk at the 19th European Sectional Conference on Atomic and Molecular Physics of Ionized Gases, Granada, Spain, 15–19 July 2008. See stacks.iop.org/PSST/18/3.

We report, for the first time, quadrupole mass spectrometry of neutral and positive ionic hydrocarbon species measured at the rf-biased substrate electrode of an inductively coupled plasma for acetylene rich C2H2 : Ar mixtures under various bias, frequency and pressure conditions. It has been observed that, irrespective of initial gas mixture, the resultant plasma is dominated by argon neutrals and ions. This is attributed to highly efficient conversion of acetylene to C2H due to the enhanced electron density compared with a standard capacitive plasma where the acetylene (neutral and ion) species remain dominant. This conversion may be crucial to film formation via inert rather than hydrocarbon ion bombardment. In addition, the transient formation of CH4 from acetylene has been discovered using IR absorption spectroscopy with time constants similar to observed pressure variations. Rate coefficients and rates for many of the reaction mechanisms, calculated using measured electron energy distribution functions and species densities, are given. These results have important application in plasma models and growth studies for hydrogenated amorphous or diamond-like carbon film deposition. Film growth under similar plasma conditions is reported in an associated paper along with ion energy distributions for important growth species.

Measurement of the energy distribution of trapped and free electrons in a current-free double layer

In the high potential plasma, upstream of the double layer, the measured electron energy distribution function (EEDF) shows a very clear change in slope at energies (εbreak) corresponding to the double layer potential drop. Electrons with lower energy are Maxwellian with a temperature of 8eV, whereas those with higher energy have a temperature of 5eV. The EEDF in the downstream plasma has a temperature of 5eV. Over the range of pressures wherein the double layer and accelerated ion beam are detected by analysis of a retarding field energy analyzer, the strength of the double layer corresponds to the energy where the slope changes in the EEDF (εbreak). We deduce that the downstream electrons come from upstream electrons that have sufficient energy to overcome the potential of the double layer, and that only a single upstream plasma source is required to maintain this phenomenon.

Plasma characteristics in a solenoidal inductive/capacitive discharge under a weak dc magnetic field

Plasma parameters such as electron temperatures, plasma potentials, and plasma densities in a solenoidal inductive/capacitive discharge immersed in a weak dc magnetic field are investigated by measuring electron energy distribution functions (EEDF). Measured EEDFs in a dc magnetic field show the typical characteristics of the local electron kinetics. The magnetic field reduces the heat flow to the electrons at the discharge center while it heats up the electrons at the discharge radial boundary. The change in the plasma potential with respect to the magnetic field at various radial positions is also discussed. It is found that the radial profile of the plasma density predominately changes with varying the magnetic field, and the magnetic field can be used to control the radial uniformity of the plasma density in the solenoidal inductive/capacitive discharge.

In situ diagnostics in plasmas of electronic-ground-state hydrogen molecules in high vibrational and rotational states by laser-induced fluorescence with vacuum-ultraviolet radiation

Detection of excited electronic-ground-state hydrogen molecules with v(") up to 13 in a magnetic multipole plasma source was performed for the first time by laser-induced fluorescence with vacuum-ultraviolet radiation. The measurements are taken after fast shutoff of the discharge current. The rovibrationally excited molecules live longer than the plasma background light so that the fluorescence light can be detected with good signal-to-noise ratio. Absolute level populations are measured as well as decay times. The theoretically predicted suprathermal population of the vibrational distribution is clearly identified. The H- density is calculated on the basis of the measured populations and the measured electron energy distribution function. It is in excellent agreement with the H- density measured by photodetachment.

Experimental Investigations on Ne/CF4 Inductively Coupled Discharges

A Langmuir probe measurement was applied to an inductively-coupled RF (13.56 MHz) Ne/CF4 discharge over a pressure range from 20 mTorr to 60 mTorr while changing the CF4 content from 5 to 20%, and keeping the power injected into the plasma at about 100 W. The measured electron energy distribution function (EEDF) under any condition deviated from the Maxwellian distribution in the energy region higher than the lowest excitation threshold energy of Ne. The average electron energy, electron density and dissociation rate coefficient estimated using the measured EEDF decreased slightly with increasing CF4 content. The EEDF and the dissociation rate coefficient in Ar/CF4 and Ne/CF4 discharges under an equally injected power were compared. The number of electrons with energy higher than the dissociation threshold energy in Ne/CF4 were much larger than that in Ar/CF4, yielding a difference in dissociation rate coefficient of one order of magnitude.

Kinetic Model of an RF Discharge in Oxygen

A kinetic model to assist in understanding the complicated physical and chemical processes occurring in an RF oxygen plasma has been developed. Ten fundamental reaction equations are considered to account for the production and loss processes of the main reactive species. The rate coefficients for the electron impact processes used in the balance equations are calculated using the measured electron energy distribution function. The calculated concentrations of O and O2 are compared with experimentally measured values for a range of pressure and RF power. The reasonable agreement between results of the model calculation and the experimental data confirms the validity of the model used.

Radial profile of diffusion-controlled He positive columns

The radial profile of electron temperature deduced from the measured electron energy distribution function (EEDF) is investigated in an axially homogeneous He positive column and compared with a theoretical one calculated from the electron energy balance equation. The radial shift of the measured EEDF can be explained by the effect of the ambipolar potential.

Experimental and theoretical investigations of DC glow discharges in argon-nitrogen mixtures

Ar-N2 mixture positive columns of direct current glow discharges are experimentally and theoretically investigated. The electron energy distribution function of the column, which has a great deviation from the Maxwellian, is affected by the amount of N2. The N2 and Ar metastable species densities estimated from the rate equations are inversely proportional to N2 content. There is a large discrepancy between experimental and theoretical electron energy distribution functions, especially for N2 positive columns. The value of pR obtained experimentally is predicted from the measured electron energy distribution function, where p is the gas pressure and R the tube radius.

Large volume radio-frequency plasma source using a magnetic line-cusp field

A large volume cylindrical radio frequency (rf)-plasma source with a magnetic line-cusp field has been developed for large scale plasma processing. In this type of plasma source, a capacity-coupled 13.56 MHz rf plasma is produced in the presence of a magnetic line-cusp field. Two versions of the plasma source have been made and tested. The first version has peripheral rf electrodes placed on the outside of the ionization chamber, and is suitable for preparing a large volume uniform plasma. This plasma source can attain a useful area of uniform plasma over a 30-cm-diam region within 10% nonuniformity. The second version is featured by parallel doughnut-plate electrodes which forms part of the chamber wall and serves as a high current source. The electron density in such a version is proportional to the rf power, being equal to 4×1010 cm−3 at 400 W. The measured electron energy distribution function showed the Maxwellian-like distribution from a drift motion in the magnetic line-cusp field.

Electron energy distribution function in a microwave discharge created by propagating surface waves

The kinetics of plasmas sustained by surface wave propagation at a frequency 2375 MHz in argon gas are studied experimentally. A simple manner for estimating the plasma parameters on the basis of general relations is given. The measured electron energy distribution functions are compared to numerical results from a kinetic modelling of surface wave produced plasma. Good agreement is found. By averaging over the measured electron energy distribution functions, the plasma macroscopic parameters (electron density and temperature, as well as frequencies of different processes initiated by electron impact) are obtained.

Gasaufheizung und Energiebilanz einer stationären hochfrequenten Ringentladung in Edelgasen

Gasheating and Energy Balance of a Stationary High Frequency Ring Discharge in Rare GasesThe energy balance of an inductively coupled high frequency (28,5 Mhz) discharge in a cylindrical vessel (11,2 cm diam.) in Ne, Ar, Kr, Xe at pressures between 0,1 and 10 Torr and at power inputs between 10 and 1000 W is investigated. The heat power transferred to the neutral gas in the stationary discharge is determined from the time behaviour of the neutral gas pressure in the afterglow period. The power measurements are completed by probe measurements of the electron density and energy distribution function.The measured electron energy distribution functions are maxwellian with a slight deficit electrons in the energy range of inelastic collisions. The electron temperatures show a rather low radial space dependence which can be explained on the basis of the local energy, balance by thermal conduction in the electron gas. The measured gas heating power is within the experimental error (factor 2) in agreement with calculations from the measured electron temperature and density under the assumption that the gas is heated by elastic electron‐atom‐collision only. A discussion of the energy balance for the total discharge indicates volume recombination losses of the ions which increase the density of the excited atoms and hence the energy losses by stepwise exitations.