Double Langmuir Probe Research Articles

Discharge cleaning of aluminum components

The rate of removal of a thin film of organic impurities during discharge cleaning in pure oxygen plasma was studied. Aluminum discs with a thickness of 2 mm and a diameter of 18 mm were first cleaned and then covered with a thin film of a polymer. The thickness of the film was estimated by AES depth profiling and was found to be about 36 nm. The samples were exposed to oxygen plasma. The plasma density and the electron temperature were measured with a double Langmuir probe and were about 1×10 16 m −3 and 6 eV, respectively. The density of neutral oxygen atoms was measured with a catalytic probe and was 3.2×10 21 m −3. Samples were exposed to the oxygen plasma in 5 s pulses in order to prevent their heating. The total treatment time was between 10 and 50 s. After plasma treatment the samples were analyzed with AES. The depth profiles showed that the thickness of the carbon layer on the surface linearly decreased with plasma exposure time. The slope of the line was about 0.8 nm/s. Taking into account the known flux of O atoms on the sample surface enabled an estimation of the probability for oxygen atoms to react with carbon on the surface. The calculated probability was about 1×10 −4.

Modelling of DC Electric Fields Induced by RF Sheath in Front of ICRF Antenna

Reducing the ICRH (ion cyclotron range frequency) antenna-plasma interaction is one of the key points for reaching very long tokamak discharges. One problem which limits such discharges, is the appearance of hot spots on the surface of the antenna: Radio Frequency (RF) sheaths modify the properties of the edge plasma by rectifying the RF potential along open magnetic field lines and can induce hot spots. This paper investigates the corrections to sheath potentials introduced by the interactions between adjacent flux tubes. Our theoretical study started from an oscillating double Langmuir probe model, in which a transverse influx of current was included. This model was confronted with 1D PIC simulations along a magnetic field line, and demonstrated that current exchanges can decrease mean potentials. A 2D electrostatic fluid code was then developed, which couples adjacent flux tubes in a poloidal cross section with collisional conductivity or polarization currents. It showed that transverse currents are able to smooth structures smaller than a characteristic size in the sheath potential maps (results for Tore Supra). These computed rectified potentials can be used to obtain the DC electric fields in front of the antenna. And then, it gives an estimate of the particle drift and the energy flux on the antenna structure, which can explain hot spots.

Characterisation of a BN magnetron sputtering process with an additional aluminium sputter electrode

The deposition of boron nitride (BN) films by r.f. magnetron sputtering of a hexagonal BN target with intended additional aluminium incorporation has been studied using various in situ plasma diagnostic techniques. The aluminium flux towards the substrates was supplied by an additional ring-shaped aluminium electrode mounted all around the circular magnetron. Langmuir double probe investigations reveal only little influence of the electrode position and electrostatic potential on the magnetron discharge. The most appropriate electrode position was found to be levelled with the anode of the magnetron. The aluminium density in the gas phase was investigated by laser-induced fluorescence and optical emission spectroscopy (OES) yielding an increase with the square of the electrode power. Moreover, a strong dependence on the Ar–N 2 gas mixture was observed which is explained by nitride formation on the surface. OES measurements also imply a significant hydrogen and boron contamination of the electrode during its power-off phases.

Ultrasonic frequency driven long plasma column

A study of argon gas stimulated at ultrasonic frequencies reveals different characteristic aspects. The plasma is created in a 50 cm tube with an effective work length of 25 cm. In order to initialize and maintain the process, plasma discharge parameters, namely, current, voltage, power, pressure, and frequency are studied and optimized. The data obtained show that in order to maintain a plasma discharge specific current, it would be sufficient to apply a higher frequency and lower voltage, whereas the current, at higher frequency and higher voltage, would be higher. Moreover the increase of the discharge voltage, frequency, and pressure increases the discharge current and consequently the discharge power. On the other hand along the half length of the tube, the plasma parameters; temperature, density, mean electron energy, and Debye length are studied and evaluated by means of a floating double Langmuir probe system. Moving from the hot electrode towards the grounded one, the electron temperature increases while the electron density decreases. The plasma temperature measured is 3–20 eV and the average plasma density is on the order 1011 cm−3, according to the pressure, the frequency, and the discharge voltage applied. The plasma source has low power consumption where, for normal operation of 25 W, the discharge voltage VD is 300 V, and the discharge current Ip is 80 mA. Away from gas breakdown, the maximum power absorbed is found to be 60 W, where VD=500 V and Ip=120 mA. A plasma phenomenon of alternatively dark and bright regions appear in the vessel, and are easily seen along the plasma tube, denoting the response of both the electron temperature and the electron density to an oscillatory behavior. The range of the applied pressure, discharge voltage, frequency, current, and power consumed inside the vessel are, respectively, 0.01–0.25 Torr, 50–500 V, 10–100 kHz, 1.5–160 mA, and 5–60 W. The formed plasma is durable, until it would be discarded. Physically it has a pale pink color that becomes brighter by increasing the discharge voltage.

Characterization and modelling of the steady state and transients of modulated hollow cathode discharges of nitric oxide

This work describes a systematic experimental study and kinetic modelling of the steady state and the transients associated with the turn-on and turn-off of a NO hollow cathode discharge. The local charge density and mean electron energy have been determined with a double Langmuir probe. Time-resolved Fourier transform infrared spectroscopy and electron bombardment quadrupole mass spectrometry with ionization by electron impact have been used to measure the gas temperature and the concentrations of the stable molecules present in the discharge: the precursor, NO, the major products, N2, O2, and the minor species NO2 and N2O. Emission spectroscopy has been employed to study the time behaviour of the very reactive nitrogen and oxygen atoms. A model based on a reduced set of kinetic equations including electron dissociation, gas-phase reactions, and gas-surface processes gives a global account of the measured data. From the time-resolved results, electron impact dissociation rate constants for some of the involved species under the conditions of the experiment are estimated. This model is compared with that obtained in previous works on N2O plasmas.

Turbulent microwave plasma thermodynamics for fundamental fluctuation modes

Microwave plasmas are generated in helium, neon, argon, krypton, and xenon at a range of microwave powers from 300 to 1800 W. A floating Langmuir double probe is employed to determine plasma electron density and temperature for all five species. The standard turbulence analysis is carried out by using time resolved neutral line emission data form these gases at a sampling rate of 100 MHz. From the Fourier power spectrum of the data, the strongest fluctuation frequency is found to be consistently the fundamental or a second harmonic of a turbulence characteristic frequency in the spectra. In all five species the strongest frequency is not influenced by increased microwave power even though other thermodynamic parameters are changed. The low chaotic dimension for all species seems independent of microwave power and of turbulent fluctuation energy. The phase space trajectories show simplicity and periodicities are consistent with the low chaotic dimension and with the peak frequencies obtained from the fluctuation spectra. The deterministic cyclic evolution of the phase space trajectories with low chaotic dimension for our plasma suggests underlying simplicity in the local turbulence physics. The average value of the strongest fluctuation frequency decreases with increasing atomic weight suggesting that the fundamental frequency is a characterizing parameter for the turbulent system.

Diagnostic studies of a low-pressure inductively coupled plasma in argon using a double Langmuir probe

The electron temperature and electron number density of a laboratory-built low-pressure inductively coupled plasma (LP-ICP) were measured using a double Langmuir probe. The probe was composed of two 0.5 mm diameter tungsten wires and a magnesia sleeve shielding the wires that could be linearly moved in the central channel of the plasma by a vacuum linear-motion feedthrough. The role of parameters was studied at a height of 25 mm above the load coil, which yielded an electron temperature of 3–6 eV and an electron number density of 2.5–5 × 1013 cm−3. Less dependence on operating factors was noticed at higher observation heights, and when water loaded on the low-pressure plasma was increased, the electron temperature and electron number density were reduced, unlike with an atmospheric ICP.

Diagnosis of dielectric barrier discharge CH 4 plasmas for diamond-like carbon film deposition

Dielectric barrier discharge (DBD) CH 4 plasmas during diamond-like carbon (DLC) film deposition have been characterized in-situ by means of optical emission spectrometry (OES), the Langmuir double probe method and molecular beam mass spectrometry (MBMS). With a 1.4 kHz, 30-kV peak voltage DBD power source, while the Pd-value (the product of CH 4 pressure P, and discharge gas spacing d) decreases from ∼14 to 4 torr mm, the measured electron temperature and the hydrogen atom excitation temperature of the CH 4 plasmas rise from ∼3.0 to 5.8 eV, and from ∼6.3×10 3 to 7.8×10 3 K, respectively. The higher electron temperature and H excitation temperature of the plasmas at smaller Pd, imply the generation of more energetic ions in the plasma sheath near the film surface, that is confirmed by the MBMS observations. The MBMS experiments also show that the major ions near the coating are CH 3 +, CH 2 +, CH + and C +, which are mainly produced through the collisions between fast CH x + ( x=1–4) and neutral CH 4 molecules in the plasma sheath region.

Measurement of Spatial Distributions of Electron Density and Electron Temperature in Direct Current Glow Discharge by Double Langmuir Probes

The spatial distributions of electron temperature and density in a dc glow discharge that is created by a pair of planar electrodes were obtained by using double Langmuir probes. The contribution of double Langmuir probes measurement is to provide a relatively quantitative tool to identify the electron distribution behavior. Electrons gain energy from the imposed electric field, and electron temperature (Te) rises very sharply from the cathode to the leading edge of the negative glow where Te reaches the maximum. In this region, the number of electrons (Ne) is relatively small and does not increase much. The accelerated electrons lose energy by ionizing gas atoms, and Te decreases rapidly from the trailing edge of the negative glow and extends to the anode. Ne was observed to increase from the cathode to the anode, which is due to the electron impact ionization and electron movement. The electron density was observed to increase with increasing discharge voltage while the electron temperature remained approximately. At 800 V and 50 mTorr argon glow discharge, Te ranged from 15 to 52 eV and Ne ranged from 6.3×106/cm3 to 3.1×108/cm3 in the DC glow discharge, and Te and Ne were dependent on the axial position.

Rapid surface modification of polyethylene in microwave and r.f.-plasmas: comparative study

The surface of polyethylene can be modified by intense oxygen plasmas in less than 200 ms. The investigation of such short treatment times is of interest for industrial applications as well as for fundamental research. The change of surface tension during the first 100 ms of a plasma treatment is a characteristic indicator for the chemical modification of the polymer surface. The correlation between surface tension, plasma parameters and treatment time is of practical interest for the layout of a plasma process and the equipment. Two plasma sources have been used: a r.f.-driven hollow cathode discharge (HCD) module and a microwave sustained slot antenna plasma source (SLAN). For each type of plasma the dependence of the surface tension on the treatment time was determined for various plasma parameters. Contact angle measurements and calculations using the harmonic mean method, were used to measure the dispersive and polar components of the surface tension of the treated samples. To trace the results back to basic plasma parameters, the plasma ion density was measured by a double Langmuir probe. The dispersive part of the surface tension is only weakly affected by the plasma treatment, in contrast to the polar part. Therefore the focus of this study was placed on the polar surface tension. All measurements show a typical exponential [∼(1−e − a· t )] dependence of the polar surface tension on the treatment time t. The increase can be characterised by a rate constant a, describing the first order kinetic of the process. Furthermore, a saturation surface tension can be defined, which is independent on the types of plasma used and of all plasma parameters, whereas the time constant depends strongly on the type of plasma. The highest rate constants were achieved with the HCD module and high r.f. power (up to 1000 W).

LINDA – the Astrid-2 Langmuir probe instrument

Abstract. The Swedish micro-satellite Astrid-2, designed for studies in magnetosperic physics, was launched into orbit on 10 December 1998 from the Russian cosmodrome Plesetsk. It was injected into a circular orbit at 1000 km and at 83 degrees inclination. The satellite carried, among other instruments, a double Langmuir Probe instrument called LINDA (Langmuir INterferometer and Density instrument for Astrid-2). The scientific goals of this instrument, as well as the technical design and possible modes of operation, are described. LINDA consists of two lightweight deployable boom systems, each carrying a small spherical probe. With these probes, separated by 2.9 meters, and in combination with a high sampling rate, it was possible to discriminate temporal structures (waves) from spatial structures. An on-board memory made it possible to collect data also at times when there was no ground contact. Plasma density and electron temperature data from all magnetic latitudes and for all seasons have been collected.Key words. Ionosphere (plasma temperature and density; plasma waves and instabilities; instruments and techniques)

Process control of a plasma treatment of wool by plasma diagnostics

Wool was treated in a large-volume microwave plasma source SLAN II [D. Korzec, F. Werner, R. Winter, J. Engemann, Scaling of microwave slot antenna (SLAN): a concept for efficient plasma generation. Plasma Sources Sci. Technol. 5 (1996) 216–234] to improve the felting tendency. To control this plasma process, the applicability of three plasma diagnostic methods, optical emission spectroscopy (OES), double Langmuir probe measurements (DLP) and mass spectrometry (MS), was investigated. The plasma is composed of a variety of species like electrons, ions, neutrals and excited atoms, radicals and molecules. Many of these may have to be taken into account for a successful treatment of the wool. OES data, which are monitoring excited particles show large fluctuations in the line intensities. The main reason for this is that the pressure does not remain constant during the plasma treatment. DLP measurements suffered the same problem. Additional problems occurred because the probe surface became contaminated during the measurement. In contrast, the relative intensities of mass lines obtained from quadrupole mass spectrometry (MS) turned out to be a good indicator for the intensity of the plasma treatment of wool.

13.56MHz hollow cathode jet matrix plasma source for large area surface coating

A novel jet matrix plasma source (JEMPS) for large area film deposition is presented. A high density plasma is produced by use of low temperature plasma jets extracted from 13.56 MHz hollow cathode discharges. A prototype of JEMPS is realized as the matrix of 7 x 11 plasma jets. The influence of gas flow, radio frequency (RF) power and pressure on the ion concentration is investigated. The typical working conditions of JEMPS are discharge gas flow 500-1000 sccm, pressure 30-100 Pa and RF power up to 400W. The ion concentration measured under such conditions by use of a double Langmuir probe amounts up to 7 x 10 10 cm -3 for argon plasma jets. Local variation of the ion concentration in a plane parallel to the anode decreases from 35% to 5% if the distance from the anode increases from 10 to 45 mm. A remote plasma deposition process is carried out with an oxygen plasma and the silicon organic monomer hexamethyldisiloxane and has a pressure range of between 30 and 100 Pa. The actual maximum deposition rate is 366 nm min -1 . The local film thickness variation is <20%.

Electron density measurements in a slot antenna microwave plasmasource by means of the plasma oscillation method

The absolute electron density in the downstream region of a 2.45 GHz microwave plasma source has been measured by the plasma oscillation method. In the plasma oscillation method a weak electron beam injected into the plasma excites electrostatic electron waves oscillating at the electron plasma frequency, which is proportional to the square root of the electron density. The plasma source is a slot antenna (SLAN) type, that is the microwave power is coupled from a ring resonator through equidistantly positioned resonant coupling slots into the plasma chamber which is made of quartz. The results of the plasma oscillation method are compared with the results of single and double Langmuir probe (LP) measurements. In Ar and Ar:O2 mixtures reasonable agreement between the two diagnostic methods is found. However, in depositing plasmas traditional LPs cannot be used with confidence. We studied Ar:O2 gas mixtures with hexamethyldisiloxane (HMDSO) added downstream for the deposition of quartz-like films. We found that even a small amount of HMDSO in the gas mixture leads to erroneous LP measurements due to probe surface coating, while the plasma oscillation method is still applicable here.

A Comparative Study of Single and Double Langmuir Probe Techniques for RF Plasma Characterization

AbstractIn this study, the plasma density and electron temperature of Radio Frequency (RF) plasmas were determined by three types of Langmuir probes, namely a conventional double probe, a single probe with RF choke and a single probe with RF choke and compensating electrode. The same plasmas were characterized by the three probes, each performing three measurements per plasma condition, in order to determine the precision of the measurement results. After performing a comparative analysis, which looked at the precision and the accuracy of these results, the conclusion is that the double probe, which has already the advantage of the simplest construction, yields the most reliable results for both capacitively and inductively coupled RF plasmas. The single probe with RF choke and compensating electrode has a similar precision as the single probe without compensating electrode, but its accuracy is better.

Diagnostics and Kinetic Modeling of a Hollow Cathode N2O Discharge

The present work describes a systematic experimental investigation of a N2O hollow cathode discharge. The local electron mean energy and density have been determined with a double Langmuir probe. Fourier transform infrared spectroscopy and mass spectrometry have been employed for the measurement of the concentration of the stable species present in the discharge. N2O, N2, O2, and NO are always identified as the main constituents of the discharge plasma. In addition, NO2 is found for the first time in a glow discharge of nitrous oxide. As a plausible explanation, a reaction of NO with oxygen atoms adsorbed on the cathode walls is proposed, although homogeneous reactions of vibrationally excited species cannot be discarded. A model based on a reduced set of kinetic equations including electron dissociation, gas-phase reactions, and gas−surface processes can give a global account of the measured data for all the experimental conditions used. The results are discussed and, when possible, compared to previous works on other types of N2O glow discharges.

Langmuir probe analysis of distributed electron cyclotron resonance silicon nitride deposition plasma

Single and double Langmuir probe analyses have been realized in the wafer region of an electron cyclotron resonance reactor in its distributed configuration. Results in nitrogen gas have shown unambiguously that two electron populations exist in this region: one with low temperature (about 1–2eV) and high density and the second with higher temperature (about 8 eV) and lower density. Measurements in silicon nitride deposition plasma (nitrogen and silane gases) have been successfully realized and have shown that these two populations are also present. Finally, we try to correlate the plasma parameters (electron temperatures and densities and ions’ energy) to the deposited film parameters (deposition rate and refractive index).

Spectroscopic studies of a magnetron sputtering discharge for boron nitride deposition

Magnetron sputtering discharges for the deposition of cubic boron nitride have been investigated using optical emission spectroscopy and Langmuir double probes. Spatially resolved measurements revealed the inhomogeneous discharge structure with respect to the distribution of excited species, electron density and electron temperature. The optical emission spectra were analysed by means of the corona model. The vibrational excitation of the nitrogen molecule was examined. The ratio of selected Ar and N 2 emission intensities provided a qualitative measure of the electron temperature, which was determined by probe measurements. However, the intensity ratios of Ar and Ar + lines did not correspond to changes in electron temperature. Varying the electron density in a wide range, it could be demonstrated that the excitation mechanisms do not remain constant, mainly due to multistep excitation. The vibrational temperature of N 2 (C 3IIΠ u) has been found to be much higher in the case of r.f. sputtering than in the d.c. mode.

Kinetics of DC Discharge Plasma Polymerization of Hexamethyldisiloxane and Pyrrole

This paper investigates DC plasma polymerization kinetics by combining plasma parameters with film deposition rate in different conditions. The monomers hexamethyldisiloxane (HMDSO) and pyrrole were used. Both single and double Langmuir probes were used to measure the plasma parameters in pulsed power and continuous discharges. In order to avoid probe tip contamination, the probe was heated. Plasma density and electron temperature are reported. The electron current wave form is obtained in pulse power conditions. From the data, a plasma polymerization model is proposed. The conclusion is that the monomer molecules and free radicals adsorbed on the substrate surface react with activated sites produced by high energy ions bombarding the film, resulting in polymerization at the film surface.

Asymmetries of Double Probe Characteristics Observed in the PSI-1

Measurements of the plasma parameters in the PSI-1 linear device, using a radially moving Langmuir double probe, are presented. The interpretation of the asymmetric I-U-characteristics is based on slight differences of the areas, plasma densities, electron temperatures, and plasma potentials of the two tips. These inhomogenities are explained by an aberration effect of the probe path with respect to the axis of the plasma column.