Pulsed RF Discharge Research Articles

Nitrogen vibrational excitation in a N2/He pulsed planar-ICP RF discharge

A pure inductive regime of pulsed RF discharge in N2–He mixtures at 0.2 Torr has been investigated by time resolved Langmuir probe and optical emission spectroscopy. A planar coil ICP with a Faraday shield has been used. Mixture compositions of 5–100% N2 and duty cycles, TON = 1–15 ms and TOFF = 15 ms, have been explored. The Langmuir probe analysis evidences a Maxwell electron energy distribution in the discharge and post-discharge, characterized by electron density and temperature that depend on both duty cycle and composition. The vibrational excitation of N2(C) state is inferred by the N2 second positive emissions. Its kinetic analysis reveals the formation of a high vibrational excitation of ground state nitrogen. A vibrational temperature higher than 10 000 K can be reached at the end of the 5 ms discharge pulse on the levels v = 0–3 of the X state. Such a high vibrational temperature slows the post-discharge relaxation of the electron temperature.

Time Resolved Actinometric Study of Pulsed RF Oxygen Discharge

The dissociation efficiency of a pulsed RF discharge (with duty cycle 1:1 and discharge duration 0.2, 1, 5, and 20 ms) in oxygen at pressures from 1 to 5 torr and effective discharge currents from 50 to 210 mA has been studied. For monitoring of the atomic oxygen density the method of actinometry has been used. In order to evaluate the absolute dissociation degree, a numerical model has been developed and actinometric coefficients were calculated. Maximal degree of the dissociation reached value of 0.05. A saturation of the dissociation degree at higher pressures was observed and discussed.

Using heated probes in plasma polymerising discharges

Using a heated probe, a technique has been developed to measure the spatial variation of the electron temperature ( T e ) plasma density ( N e ), floating ( V f ), and plasma potentials ( V p ), and the electron energy distribution function (eedf) in low-pressure polymerising plasma. During the non-data acquisition times, the probe was heated using an external current, so minimising deposition of an insulating layer on its surface. Typically the insulating film deposition rate was between 2 and 4 nm s −1 depending on the discharge conditions. To obtain a Langmuir probe characteristic (and derive the quantities N e , T e , V f and eedf), the heating current was switched off and the data collected over a short time (10 to 30 s). By heating the probe to strong electron emission, it was also possible to obtain an accurate measurement of V p . The technique has been applied to a new two-stage reactor (source and diffusion chambers separated by a mesh) in which acrylic acid is plasma polymerised to form thin films with functional surface chemistry at low pressure (5.2 Pa). The probe results show that in the polymerising chamber, with increasing distance from the mesh, (15 to 70 mm), N e fall from 1.7×10 14 to 2×10 13 m −3 and T e falls from 3.7 to 1.3 eV. However, at a fixed distance of 70 mm, with increasing discharge power, T e remains constant, while N e increases over an order of magnitude. The low plasma densities and low electron temperatures obtainable, give rise to conditions, similar to those obtained in pulsed RF discharges, and therefore favourable conditions for the control of surface functionality in the deposited film.

Charge carrier dynamics in a pulsed inductive RF discharge in oxygen

We report on the spatio-temporal behaviour of negative oxygen ions in a pulsed inductively coupled RF excited oxygen discharge in the gaseous electronics conference cell. The temporal behaviour of the positive and negative charge carriers in the afterglow of the discharge allows identification of the dominant loss processes of negative ions. The results are in good agreement with model calculations. The negative ions are trapped for a long time in the bulk plasma without diffusion to the walls. Negative ions can diffuse to the walls only after complete decay of the electron component and formation of an ion–ion plasma. We can identify a high fraction of negative oxygen molecular ions that are generated during this phase in collisions of negative atomic oxygen ions with metastable oxygen molecules. Such a high fraction of molecular negative ions has not been observed so far in continuous discharges.

Using an afterglow plasma to modify polystyrene surfaces in pulsed radio frequency (RF) argon discharges

A systematic study has been made to characterise a pulsed RF discharge used for the plasma treatment of polymers. Using a time-resolved Langmuir probe and a time-resolved retarding field energy analyser (RFA) the electron temperature and density, plasma potential, ion energy distribution function (IEDF) and electron energy distribution function (EEDF) has been measured. The source was pulsed for a range of frequencies from 100 Hz to 1 KHz at various duty cycles from 20 to 80%. A base pressure less than 10 −5 Torr was achieved to decrease the level of impurities in the chamber. The electron temperature ( T e ) decreases from 5 eV to less than 0.5 eV in afterglow. The cooling rate of T e depends on the pressure, power and duty cycle. By pulsing a biased grid synchronously with the pulsed RF, ions are selected from different time of the plasma and allowed to impinge on a polymer surface. Modified oxygen to carbon (O/C) ratio in the upper layer of polystyrene has been measured by X-ray photon emission spectroscopy (XPS) for different energy and flux of plasma ions. These results show that the treatment is either insensitive to ion energy or dominated by the UV in the discharge.

Diamond deposition with chlorinated methanes: investigation of the plasma chemistry

Methane and chlorinated methanes (CH 4, CH 3Cl, CH 2Cl 2, CHCl 3, CCl 4) were diluted in hydrogen and were decomposed in an electrodeless pulsed RF discharge sustained in a quartz glass tube. From the active plasma zone a small amount of the process gas was extracted by a helium flushed probe system, transported to and detected by a calibrated quadrupole mass spectrometer. The gas phase chemistry in the reactor was also modelled by CHEMKIN computer code. A reaction mechanism for the CHCl system, which consists of 158 reactions and 40 species, was used to model the complex plasma chemistry. It turned out that in the plasma the chlorinated methanes are decomposed very quickly to the main stable species HCl, CH 4 and C 2H 2. Chlorinated hydrocarbon radicals are only present within a few millisecond after the initial contact of the precursor with the plasma. At steady state, the radicals CH 3, Cl and H are most abundant. CH 4, C 2H 2, and also smaller amounts of C 2H 4 and C 2H 6 are in a quasi equilibrium. The chlorinated hydrocarbons were close to the detection limit.

The characterization of structure-tailored plasma films deposited from the pulsed RF discharge

Pulsed RF plasma polymerization using vinyl acetic acid as a prototype precursor was investigated in order to evaluate the applicability of the pulsed RF discharge to obtain plasma polymers with less cross-linked structure and high degree retention of the groups in the starting monomer. The chemical structure and the surface morphology of the polymerized vinyl acetic acid were characterized by Fourier transform infrared (FTIR) spectroscopy, X-ray photoelectron spectroscope (XPS) and scanning electron microscope (SEM). At all other plasma parameters remaining constant, the chemical structure was observed to vary with the plasma duty cycles. The FTIR results showed that more carboxylic groups could be ‘retained’ with the decreasing of the duty cycles. The XPS results were consistent with the FTIR measurements. Surface energy measurements indicated that the plasma films were hydrophilic. The plasma polymerized films at continuous discharge mode show higher thermal stability than the films polymerized at pulsed discharge mode. But the later is less cross-linked. So it is possible to control the chemical composition of the plasma film with the same surface functional groups as its monomer being ‘retained’ by pulse plasma technologies. Some regular graft ‘lightning’ network patterns were interestingly found in the plasma films kept at room temperature for some time by SEM. It was estimated that the mechanism of the plasma polymerization is different along the ‘lightning’ network and on the valley of the pattern.

Formation of hydrogenated amorphous carbon films by plasma based ion implantation system applying RF and negative high voltage pulses through single feedthrough

A plasma-based ion implantation system that can apply both radio frequency wave (RF) and negative high voltage pulses through single feedthrough has been developed. The system was successfully applied to the formation of hydrogenated amorphous carbon (a-C:H) thin films. The a-C:H films were deposited by capacitively coupled continuous wave (CW) RF discharge of pure methane (CH 4) or mixture of CH 4 and Ar. Ion implantation by negative high voltage pulse bias was also demonstrated under the pulsed RF discharge of CH 4. Mechanical properties of the deposited a-C:H films were measured by nano indentation. Maximum hardness and elastic modulus of the film was 23 and 220 GPa, respectively.

The N2(A3Σu+) energy transfer toOH(A2Σ+) in low-pressure pulsed RF discharges

In this paper we demonstrate that the excitation ofOH(A2Σ+) in a pulsed N2-H2O RF discharge is produced mainly by thereaction N2(A3Σu+,v) + OH(X2Σ+) →N2(X1Σ+) + OH(A2Σ+,v'). We thenobtain in the post-discharge the quasi-nascent rotational distribution ofOH(A2Σ+,v = 0) produced by such an energy transfer reaction, showingthe probable presence of three main de-excitation paths for N2(A) in theprocess. Finally, we propose to use the knowledge gained in this study todesign a N2-H2O pulsed RF discharge as a calibrated source of OH.

Spectrum and Energy Levels of Singly-Ionized Mercury (Hg II)

The spectrum of singly-ionized mercury (Hg II) has been observed from 500 Å to 9880 Å with sliding sparks and pulsed-RF discharges on 10.7-m grating spectrographs in vacuum and air. About 500 lines have been classified as transitions between 114 energy levels. In addition to revising the existing system of 5d10nl, 5d96s2, and 5d96s6p energy levels, we have located new levels of the 5d86s26p, 5d96s7p, 5d96s5f, 5d96s7s, 5d96s6d, and 5d96p2 configurations, which occur in the region of the ionization limit. Many of the observed configurations were theoretically interpreted by means of Hartree-Fock calculations and least-squares fits of the energy parameters to the observed levels. The fitted parameters were used to calculate oscillator strengths for most of the classified lines. The ionization energy was revised to 151284.4±0.3 cm-1 (18.75687±0.00004 eV).

Excitation and decay of N2(B 3Πg,v) states in a pulsed discharge: Kinetics of electrons and long-lived species

The vibrational excitation and decay of N2(B 3Πg) state has been investigated in a N2 pulsed rf discharge. The effect of the pulsing frequency and the duty cycle on the N2 (B,v=1–12) vibrational distribution, obtained from the 1PG spectra taken at different times in discharge and afterglow, has been examined in the N2 pressure range 2.4 mTorr–1.5 Torr. The measured (B,v) distributions have been analyzed by a steady-state kinetic model taking into account the main excitation processes, like the electron impact from N2(X 1Σg+,v) and N2(A 3Σu+,v) states, the associative excitation of N2(X,v) with N2(A,v), the pooling by N2(A,v) molecules, the atomic recombination of N(4S), as well as the quenching processes. For the various processes, experimental state-to-state rate coefficients from the literature and/or calculated data sets have been used. Measured N2(A,v) and electron energy distribution functions, and estimated N2(X,v) distributions have been used as input data for the model. The model satisfactorily reproduces almost all the measured distributions. The kinetic analysis evidences that the shape of the (B,v=1–12) distribution depends on the competition between the different processes and in turn on the time variation of both density and the degree of internal excitation of the reaction species with discharge repetition rate, duty cycle, and pressure. In general, in the discharge the electron impact is always the predominant excitation mechanism, while the processes involving long-lived species: N2(A,v) and N2(X,v) are important in the postdischarge regime. Under conditions of high metastable density, the associative collision processes contribute to (B,v) excitation in the discharge, and the electron impact process from the A and X states gives a non-negligible contribution to the low v levels in the millisecond time scale afterglow. An analysis of the B quenching is carried out.

Atmospheric Plasma Sterilization and Deodorization of Dielectric Surfaces

A method is presented for rapid and uniform sterilization and deodorization of dielectric surfaces. The technology is applicable to the inside surface of PET or glass bottles, polymer caps, plastic tubes, etc. The treatment is based on a pulsed RF discharge in air at atmospheric pressure (eventually with addition of argon) creating a nonequilibrium plasma on the treated surface. The plasma effectively destroys microorganisms in vegetative or sporulent form. It also slightly etches the polymeric material, removing some atomic layers and, thereby, cleaning it from aromatic organic components (deodorization). The process is short: PET bottles 1.5 L, in particular, can be treated in about 20 msec. The results of surface analysis and microbiological, chromatography, and spectroscopy tests are discussed. A device has been developed and integrated into an industrial-filling machine for online sterilization and deodorization of the inside surface of PET bottles before filling, and for sterilization of caps and bottle necks before seaming. It allows cold asceptic filling at a rate of 36,000 bottles per hour.

Experimental studies on elementary kinetics in N2-O2 pulseddischarges

In this paper we discuss `early afterglow' studies on elementary kinetics in a pulsed rf discharge in N2 and N2-O2 mixtures. In particular we review the following topics: (a) LIF measurements of N2(A 3u+) density and v = 0-9 vibrational distribution; (b) measurement of N2(A 3u+, v = 2-7) quenching rate constants by O2, O and NO; (c) study of the NO(A 2+) excitation; (d) N2(B 3g) vibrational distribution measurement and analysis of its excitation processes by means of a kinetic model. New results are moreover presented and discussed: (e) two-photon LIF (TALIF) measurements of O-atom density and loss rate; (f) LIF measurements of NO density.

LIF measurement of population density in a pulsed rf discharge

A laser-induced fluorescence (LIF) technique has been applied to the measurement of population density in a pulsed parallel-plates rf discharge at 0.1 Torr pressure. The method is based on a single rotational line excitation-detection LIF scheme on the first positive band system of . This approach makes the correct application of a standard model of three-level rate equations to the description of the LIF process for the treatment of experimental results possible. Conditions for true single-line operation using a line-width pulsed tunable laser are discussed and applied to the measurement. The measured density value is then used for the absolute density calibration of a previous relative vibrational distribution of -9) obtained by a non-single-line excitation-detection LIF measurement.

The NO(A) excitation mechanism in a - pulsed RF discharge

Time-resolved laser-induced fluorescence detection of metastable and NO and time-resolved emission spectroscopy of the -band of NO have been applied to a - RF pulsed discharge in order to show that the process is responsible for NO(A) excitation in discharge systems of this kind. Highly resolved spectra obtained under such conditions have revealed that there is a non-Boltzmann rotational distribution in the ) state with a large excitation of levels J>20.

Surface Reaction Kinetics of CH 3 in CH 4 RF Discharge Studied by Time-Resolved Threshold Ionization Mass Spectrometry

The surface loss probability β of CH3 radicals on a-C:H surface is determined by time-resolved threshold ionization mass spectrometry in pulsed RF discharges. In the post-discharge, the β value decreases from 0.011–0.015 to 0.001–0.002 in about 10 ms. This time evolution suggests that the loss probability depends on incoming ion and radical fluxes on the surface. We present a simple model of the surface reaction kinetics in which the number of active chemisorption sites on the surface is determined by the balance between their production, due to ions and/or H, and their annihilation, due to CH3 radicals. The model describes fairly well the observed decay of the CH3 radical density and the time evolution of β determined experimentally in the afterglow. The maximum contribution of the CH3 radicals to the deposition rate of a-C:H films was deduced to be about 60% of the total deposition rate determined experimentally.

Ion energy distributions in oxygen and argon in a pulsed mode RF discharge

The positive and negative ion energy distributions were measured at the grounded electrode of a square wave modulated RF discharge in O 2 and Ar at between 1.5 and 5 Pa gas pressure. The resulting ion energy distributions were strongly affected by the modulation frequency according to the existence of different discharge periods during the modulation cycle. These discharge periods were directly represented in the energy distribution of the positive ions which is determined by the plasma sheath voltage in front of the grounded electrode. Negative ions generated in the plasma sheath at the RF electrode were accelerated towards the bulk plasma and detected with a high energy at the opposite grounded electrode. Additionally, during the afterglow with decaying sheath potential, low energy negative ions reached the electrodes. The intensity ratio between the two groups of negative ions allowed conclusions on the negative ion generation process.

Plasma polymerized acetylene thin film by pulsed discharge

Plasma polymerization of acetylene was performed by using a pulsed rf discharge. Capacitively coupled discharge was initiated in a box type reactor at a uniform flow of an acetylene/argon mixture gas. A 2–10 nm scale ultrathin hydrocarbon film was deposited on a silicon wafer. The distribution of thickness was measured by an eliipsometer. The thickness increased nonlinearly with distance from the monomer inlet. This means that secondary gas phase products took part in the film formation.

A High-Power Q-Switched CO2 Laser Using Intense Pulsed RF Discharge Excitation

A compact high-power Q-switched CO2 laser has been developed using a combination of intense pulsed rf (13.56 MHz) discharge excitation and a high-speed rotating Q-switch chopper. Performance characteristics have been investigated as functions of Q-switching velocity, input discharge energy density and laser gas composition. A pulse peak power of 300 kW and an average power of 740 W at 10 kHz pulse repetition frequency have been obtained simultaneously for a Q-switched laser based on a CO2 laser with 1.1 kW average power. By decreasing the repetition frequency to 4 kHz, the peak power was enhanced to above 500 kW. An analytical model describing the unique features of the Q-switched CO2 laser has also been developed for quantitative evaluation of the laser performance. Good agreement was obtained between the theoretical model and experimental results.

パルス化RF放電によるプラズマ重合アセチレン超薄膜の生成モデル

Plasma polymerization by using a pulsed rf discharge was performed for preparation of ultra thin organic film. The discharge capacitively coupled was initiated in a box-type reactor at an uniform flow of an acetylene/argon mixture gas. 2-10 nm scale ultra thin hydrocarbon film was deposited on a silicon wafer. The surface topography of the film examined by AFM appears to be pinhole free. The distribution of thickness measured by an elipsometer suggested the reaction in gas phase is significant. Therefore, the theoretical calculation was performed by using a model of radical reaction in the gas phase, and the obtained equations were fitted well to the results of experiments.