Double Langmuir Probe Research Articles

SYMMETRIC AND ASYMMETRIC DOUBLE LANGMUIR PROBES CHARACTERIZATION OF RADIO FREQUENCY INDUCTIVLEY COUPLED NITROGEN PLASMA

The symmetric and asymmetric double Langmuir probe systems with their necessary driving circuits are developed for characterization of low pressure inductively coupled nitrogen plasma, generated and sustained with 13.56 MHz RF source and an automatic impedance matching network. First of all, the plasma parameters such as ion saturation current, electron temperature and electron number density are determined with symmetric double probe system at different input RF powers, filling gas pressures and radial distance Received 4 March 2011, Accepted 28 March 2011, Scheduled 30 March 2011 Corresponding author: Muhammad Yasin Naz (yasin603@yahoo.com).

HBr/Ar/CHF3혼합가스를 이용한 ZnO 박막의 유도결합 플라즈마 식각

In this work, the etching characteristics of ZnO thin films were investigated using an inductively coupled plasma(ICP) of HBr/Ar/<TEX>$CHF_3$</TEX> gas mixtures. The plasma characteristics were analyzed by a quadrupole mass spectrometer (QMS) and double langmuir probe (DLP). The surface reaction of the ZnO thin films was investigated using X-ray photoelectron spectroscopy (XPS). The etch rate of ZnO was measured as a function of the <TEX>$CHF_3$</TEX> mixing ratio in the range of 0-15% in an HBr:Ar=5:2 plasma at a fixed gas pressure (6mTorr), input power (700 W), bias power (200 W) and total gas flow rate(50sccm). The etch rate of the ZnO films decreased with increasing <TEX>$CHF_3$</TEX> fraction due to the etch-blocking polymer layer formation.

Effects of ionic liquid electrode on pulse discharge plasmas in the wide range of gas pressures

Gas–liquid interfacial pulse discharge plasmas are generated in the wide range of gas pressures, where an ionic liquid is used as the liquid electrode. By analyzing the characteristics of discharge voltage and current, the discharge mechanisms at low and high pressures are found to be dominated by secondary electron emission and first Townsend ionization, respectively. Therefore, the discharge properties at low and high pressures are mainly determined by the cathode material and the discharge gas type, respectively. Furthermore, the plasma properties are investigated by a double Langmuir probe. The density of the positive pulse plasma is found to be much smaller than that of the negative pulse plasma, although the discharge voltage and current of the negative and positive pulse plasmas are of the same order of magnitude. The positive pulse discharge plasma is considered to quickly diffuse onto the chamber wall from the radially central region due to its high plasma potential compared with that in the peripheral region.

Actinometry Measurement of Dissociation Degrees of Nitrogen and Oxygen in N2–O2 Microwave Discharge Plasma

The dissociation degrees of N2 and O2 are examined in a nitrogen–oxygen mixed microwave discharge plasma in a cylindrical quartz tube of 26 mm inner diameter with a discharge pressure of 0.5–1.0 Torr and a microwave power of 600 W by the actinometry method. We measured the electron temperature and density with a Langmuir double probe, while the vibrational and rotational temperatures of the first and second positive bands of N2 were measured by optical emission spectroscopy. Even when the line intensity of atomic nitrogen was weak and partly coincided with the high-intensity band spectrum of the first positive system due to its small dissociation degree, the actinometry method was found to be feasible when the first positive band spectrum, calculated as a function of the rotational and vibrational temperatures, was subtracted from that observed experimentally. It was found that the dissociation degrees of both N2 and O2 increase with the molar ratio of nitrogen in the mixed N2–O2 discharge gas for the same total discharge pressure. The experimental results are discussed by comparison with a simple numerical model based on chemical kinetics in the plasma. It was found that the dissociation of oxygen molecules is enhanced by the collision with excited nitrogen molecules, particularly those with metastable states, whereas that of nitrogen is suppressed by an admixture of oxygen molecules due to the chemical quenching processes of nitrogen atoms.

Etching characteristics of Pb(Zr,Ti)O3, Pt, SiO2 and Si3N4 in an inductively coupled HBr/Ar plasma

An investigation of the etching characteristics of Pb(Zr,Ti)O3 (PZT), Pt, SiO2 and Si3N4 in an inductively coupled HBr/Ar plasma as functions of gas mixing ratio at a constant gas pressure (4 mTorr), total gas flow rate (40 sccm), input power (700 W) and bias power (300 W) was carried out. It was found that the PZT etching rate exhibits a maximum at 60–70% Ar, while the highest PZT/Si3N4, PZT/SiO2 and PZT/Platinum (Pt) etch selectivities correspond to 40%, 60% and 100% Ar, respectively. Plasma diagnostics by a double Langmuir probe and a global (zero-dimensional) plasma model provided data on plasma parameters, densities and fluxes of plasma active species. It was proposed that, in HBr-rich plasmas, the PZT etching process appears in the transitional etch regime of the ion-assisted chemical reaction and the non-monotonic behavior of the PZT etching rate results from the concurrence of both chemical and physical etch pathways.

Langmuir Probe and Optical Emission Spectroscopy Studies of Low-Pressure Gas Mixture of CO2 and N2

Optical emission spectroscopy was used to study a gas mixture glow discharge of CO2 and N2 at a total pressure of 1.2 Torr, a power of 100 W and a flow of 16.5 L/min. The emission bands were measured in the wavelength range of 200 nm to 900 nm. The principal species observed were O2+ (A2Πu → X2Πg), CO+ (A2Π → X2Σ), N2+ (B2Σu+ → X2Σg+), CO2+ (A2Π → X2Π), N2(C3Πu → B3Πg), O2(b1Σg+ → X3Σg−), and CO (a′3Σ → a3Π). The behavior of the band intensities as a function of the N2 percentage is consistent with recent Monte Carlo simulations. The electron temperature and ion density were determined by a double Langmuir probe. The electron temperature was found in the range of 1.55 eV to 2.93 eV, and the electron concentration in the order of 1010 cm−3. The electron temperature and ion density at pure N2 and pure CO2 agree with previous measurements.

Etching Characteristics of In2O3 and SnO2 Thin Films in an Inductively Coupled HBr/Ar Plasma: Effects of Gas Mixing Ratio and Bias Power

An investigation of the etching characteristics and mechanisms of both In2O3 and SnO2 in a HBr/Ar inductively coupled plasma was carried out. The etching rates were measured in the range of 0–100% Ar and 100–300 W bias power at a fixed gas flow rate (40 sccm), total gas pressure (6 mTorr), and input power (700 W). The plasma parameters and composition were determined using a combination of plasma diagnostics by a double Langmuir probe and a global (zero-dimensional) plasma model. It was found that both In2O3 and SnO2 etching rates are mainly controlled by the Br atom flux, with some sensitivity to the ion energy flux corresponding to the transitional regimes of the ion-assisted chemical reaction.

Etch mechanism of In2O3 and SnO2 thin films in HBr-based inductively coupled plasmas

The investigations of etch characteristics and mechanisms for both In2O3 and SnO2 thin films in the HBr-based inductively coupled plasmas were carried out. The etch rates were measured as functions of gas mixing ratio (0%–100% Ar), input power (400–700 W), and gas pressure (4–10 mTorr) at fixed bias power (200 W) and gas flow rate [40 SCCM (SCCM denotes cubic centimeter per minute at STP)]. Plasma parameters and composition were determined using a combination of plasma diagnostics by double Langmuir probe and global (zero-dimensional) plasma model. The correlations between the behaviors of etch rates and fluxes of plasma active species allow one to infer both In2O3 and SnO2 etch mechanisms as the transitional regime of ion-assisted chemical reaction, which is controlled by neutral and charged fluxes together.

Initial Plasma Confinement Experiments in Tokamak-Helical Hybrid Device TOKASTAR-2

Tokamak and helical systems have been widely recognized as efficient toroidal magnetic plasma confinement devices. TOKASTAR-2 [1] is a new plasma confinement device which has both confinement properties. There are outer helical field coils with the toroidal period n=2 or 1 outside of eight toroidal field coils. One of main purpose of this experiment is to evaluate effects of external helical field application on tokamak plasma confinement. Present status of TOKASTAR-2 is in the phase that electron cyclotron resonance of radio frequency wave creates plasmas. In this conference, analysis of magnetic field surface, plasma production near electron cyclotron heating (ECH) resonance layer, estimation of electron density and temperature, and radial density profile in this device are presented. Electron density and temperature were measured using single or double Langmuir probe. Central density and temperature are estimated in order of 10m and 10eV respectively. In the measurement of plasma density profile in space, using double probe, plasma density profile is analyzed assuming that measured probe current is proportional to plasma density at respective measuring position of probe. Figure 1 shows temporal evolution of probe current which were measured at each position of radial direction. In this case, simple toroidal magnetic field was applied and plasma current was not induced. Figures (a),(b),(c) and (d) of Fig. 1 denote temporal evolution of probe current at major radius R=17, 13, 9, 7cm, respectively. As shown in this figure, during 2~3 ms, peak of probe signal is observed at the inner radial position at the early stage. The peak position goes to the outside gradually. During 3~5ms, signal level is almost constant at R=17cm. Moreover during 5~6ms, peak of signal is observed at inside direction. Therefore, initiated plasma at the inside position moves outward, and return to the inside, which corresponds to the movement of ECH layer. Analysis of magnetic field surface and ECH resonance layer position in the poloidal cross section is being carried out by using computer calculation codes. New experimental results of outer helical field application will be also presented in the poster.

Etching characteristics and mechanism of indium tin oxide films in an inductively coupled HBr∕Ar plasma

The investigations of etch characteristics and mechanisms for indium tin oxide (In2O3)0.9:(SnO2)0.1 (ITO) thin films using HBr∕Ar inductively coupled plasma were carried out. The ITO etch rate was measured in the range of 0%–100% Ar in the HBr∕Ar mixture at fixed gas pressure (6mTorr), input power (700W), and bias power (200W). Plasma parameters and composition were examined with a combination of plasma diagnostics by double Langmuir probe and global (zero-dimensional) plasma model. It was found that the ITO etch rate follows the behavior of Br atom flux but contradicts with that for H atoms and positive ions. This suggests that the ITO etch process is not limited by the ion-surface interaction kinetics and appears in the reaction-rate-limited etch regime with the Br atoms as the main chemically active species.

Diagnostics and modeling of CH4–CO2 plasmas for nanosmooth diamond deposition: Comparison to experimental data

Microwave plasma-assisted chemical vapor deposition of very smooth diamond coatings is an important process for various applications including mechanical and micromechanical systems and acoustic wave devices. Nanosmooth coatings have been deposited from CH4–CO2 gas mixtures at moderate temperature, the order of 600 °C. In order to increase the knowledge of the process and the control of the final characteristics of the films, a modeling of these plasmas is necessary. This has been carried out here from the prior determination of the plasma parameters. Optical emission spectroscopy was used in order to determine the gas kinetic temperature. Microwave interferometry and Langmuir double probe were used to determine the electron density and the electron temperature, respectively. All these experimental data have been obtained for a wide range of external parameters, such as the inlet composition, the pressure, the gas flow rate, and the power injected in the plasma. Then modeling of CH4–CO2 plasmas was developed by coupling chemical kinetics with a two-dimensional description of hydrodynamics and a surface-wall recombination of main radicals. The kinetic description of the CH4–CO2 plasmas was done by combining a specific mechanism of dissociation by electrons to a slightly modified version of a combustion mechanism for neutral-neutral interactions. This model has been validated by comparing the calculated species concentrations and the experimental results obtained by molecular beam mass spectrometry as a function of various external parameters. The influence of the inlet composition at three microwave power densities has been especially emphasized here. The calculations are in good agreement with the experimental results. It is shown that among the various parameters that influence the diamond growth from CH4–CO2 plasmas, the power density injected in the plasma is very important as it changes strongly the degree of completion of the chemical system and then the deposition conditions.

Dual comb-type electrodes as a plasma source for very high frequency plasma enhanced chemical vapor deposition

Dual comb-type electrodes were developed as a plasma source in very high frequency (VHF) plasma enhanced chemical vapor deposition system for uniform deposition of silicon films. Two VHF powers introduced to each electrode produced parallel plasma bands, and their positions could be changed by manipulating the phase difference between the supplied VHF waves. Excitation frequency was 80 MHz. The maximum plasma density using this plasma source was 1.5 × 10 10/cm 3 and the electron temperature was around 2 eV with input power of 2.5 kW, which were measured by double tip Langmuir probe. The uniformity of deposition rate under ± 13% was achieved on 1 m 2 area with optimal plasma conditions.

Etching characteristics and mechanism of ZnO thin films in inductively coupled HBr/Ar plasma

The etching characteristics and mechanisms of ZnO thin films in an HBr/Ar inductively coupled plasma were investigated. The etch rate of ZnO was measured as a function of the HBr/Ar mixing ratio in the range of 0–100% Ar at a fixed gas pressure (6 mTorr), input power (900 W) and bias power (200 W). The plasma diagnostics were performed by double Langmuir probe measurements and quadrupole mass spectrometry. A global (0-dimensional) plasma model was used to obtain the data on the densities and fluxes of the active species. It was found that the etch rate of ZnO is proportional to the flux of Br atoms, but inversely proportional to those of the H atoms and positive ions. This suggests that the ZnO etch process is not limited by the ion-surface interaction kinetics and that the Br atoms are the main chemically-active species.

Comparison between particle simulation and measurements of a stationary plasma thruster plume

This article is attempted to study the plume characteristics of a stationary plasma thruster (SPT). First, electron density, electron temperature, and current density of a SPT plume in a chamber are measured experimentally by a double Langmuir probe. Then, a numerical model based on particle-in-cell and direct simulation Monte Carlo hybrid method is employed to simulate the flow field. The simulation is verified by comparing the computational results and the experimental data. The research indicates that the electron temperature of the flow field is ∼ 2 eV and the electron number density is ∼2.5×1016−5×1015 m−3 at the centre-line and at a distance of 0.3—1.0 m downstream of the thruster exit. The divergence angle of the researched SPT plume is ∼30° and the influence of backpressure is observable at the wings of the flow field.

Electrical and spectroscopic characterizations of a low pressure argon discharge created by a broad-band helical coupling device

This paper presents the spatial characterization of Ar plasma induced by a broad-band helical coupling device at low pressure (15 Pa). The electric field analysis outside the helix but within the coupling device is carried out by electrostatic probe for several excitation frequencies ranging from 13.56 to 433 MHz. The results prove that this helical cavity behaves as a slow-wave plasma source. At 13.56 MHz, the spatial investigation of electric field with and without plasma led to show a flat profile along the helix structure. At this frequency, the plasma is studied by spatially resolved emission spectroscopy and by means of a double Langmuir probe in order to evaluate the densities of some characteristic species (electron and Ar emitter) and the electron and gas temperatures. These temperatures are invariants along the discharge with average values about 400 ± 50 K and 2.3 ± 0.6 eV for gas and electron, respectively. Consequently, the electron energy distribution function and argon ground state density are also constant along the discharge. No variation of the argon metastable density, estimated by means of a simplified kinetic model, could be shown and an average value equal to 9 × 1017 m−3 is obtained. In contrast, the density profiles of electron and Ar emitter show a minimum situated in the middle of the helical structure and two maxima located at the positions where the RF power is transmitted to the gas and at the end of the helix. The electron density values vary from 3 × 1016 to 1017 m−3. According to the low pressure conditions and the flat evolution of the electric field, these hollow shaped density profiles can be explained as a consequence of a non-linear phenomenon.

Automatic measurements of plasma parameters in the PUPR mirror and cusp plasma machine

This paper presents an ongoing effort to develop an automatic measurement system for plasma diagnostics at the Plasma Engineering Laboratory of the Polytechnic University of Puerto Rico (PUPR), along with an example of its operation. The system is intended to be used with electrostatic probes such as single and double Langmuir probes, emissive probes, ion and electron energy analyzers, etc. The automatic measurement system includes automatic positioning of the probes inside the plasma chamber, automatic voltage sweep of the probes for each position, and automatic analysis of the probe I–V characteristic. The results of measurements obtained by using this automatic measurement system during a particular experiment are shown and compared with a traditional method with satisfactory results.

Inductive plasma source for the ion treatment of AISI-304 SS

The design and construction of a simple inductive plasma source is described as constituted by an evacuated Pyrex glass cylinder reactor with 190 mm inner diameter and 500 mm length. This discharge vessel is coaxially surrounded by a cylindrically wound antenna, 240 mm in diameter, made of 3.2 mm wide copper wire. The antenna is supplied by a 13.56 MHz RF generator whose resulting electric field is able to create the plasma. When nitrogen is admitted to the vessel, the plasma generation takes place within the 0.1–50 Pa work pressure and 300–600 W RF power. The plasma density has been established by double Langmuir probes between 3.2 × 1015 and 2.4 × 1018 m−3. This inductive plasma set up is meant to modify the surface of AISI-304 stainless steel by means of ion deposition, thanks to the sample bias provided by an external - 400 V dc supply, in order to improve the steel hardness without compromising its corrosion resistance. Once accelerated by the negative bias, the plasma ions impinge on the sample nitriding it by diffusion. The treated samples were characterized by x-ray diffraction (XRD) indicating the formation of the expanded gamma phase, by scanning electron microscopy (SEM) providing the atomic percentages of nitrogen, and by microhardness (HV) measurement.

Study of Plume Characteristics of a Stationary Plasma Thruster

Electron density and temperature of the plume are measured by a double Langmuir probe in an experimental chamber. A numerical model based on both particle-in-cell scheme and direct simulation Monte Carlo hybrid method is developed to simulate the flow field of plume. The equation for plasma potential is solved by alternative direction implicit technique. The simulation is verified by comparing the computational results with the measured data. The study indicates that the electron temperature of flow field is about 2 eV and the electron density is about 2.5 × 1016 ∼ 5 × 1015 m−3 at the central line with a distance of 0.3 ∼ 1.0 m downstream of the thruster exit. The model can well predict the flow field parameters of the steady plume. The efforts of this paper are referable for further investigation.

Plasma Immersion Ion Implantation With Lithium Atoms

A new method was developed to produce lithium plasma for plasma immersion ion implantation. Initially, an argon glow discharge with operation pressure ranging from 2 times 10-1 to 1 mbar is generated by negatively polarizing an electrode from -400 to -1500 V. Small pieces of metallic lithium that are 99.9% pure fill the top of a conic crucible, with a depth of 2 cm, in electric contact with the electrode. Argon ions from the plasma are used to bombard this target, where heat is created by the momentum transfer from the impacting ions to the crucible. By controlling the operation pressure and the electrode voltage polarization, it is possible to easily heat the crucible to temperatures above the lithium melting point (180degC), causing its evaporation. Lithium atoms are then ionized, mainly due to collisions, with argon ions moving toward the crucible. Double Langmuir probe measurements indicated variation in the density of the discharge from 4 times 109 cm-3 to 1010 cm-3 after lithium evaporation. Silicon wafer pieces immersed in this mixed plasma were submitted to repetitive negative high-voltage pulses (3 kV/6 mus/2.5 kHz) to accelerate plasma ions. High strain in the treated layers was measured by high-resolution X-ray diffraction. Photoluminescence intensity increased after annealing. X-ray photoelectron spectrometry measurement revealed lithium implantation in silicon with an atomic concentration of 78% on the top surface and a penetration depth of about 75 nm.

Study on plasma discharge parameters in double-glow plasma surface alloying furnace

Double Langmuir probe technique was used for plasma diagnostics of discharge process in double-glow plasma alloying furnace. Investigated that the effect of changes of voltage of the source pole and air pressure on plasma parameters. Results show that plasma density increases, electron temperature decreases and potential of the probe decreases, with the rise of voltage of the source pole under condition of fixed cathode potential and fixed air pressure. During increasing of air pressure in furnace, electron temperature decreases and probe potential increases. The change of probe potential is smart at start of the measuring and probe potential goes to stabilization only when degas process goes to finishing.