Wide Range Of Gas Pressures Research Articles

Backward stimulated Raman scattering on the Q12(1) transition in vibrationally excited hydrogen molecules

The results are reported of an experimental investigation of the backward stimulated Raman scattering on the vibrational transitions Q01(1) and Q12(1) in hydrogen molecules subjected to nonmonochromatic laser pumping. The experiments were carried out in a wide range of gas pressures at pump powers corresponding to an exponential enhancement of the intensities of the Stokes components. Under the conditions of tight focusing of the pump radiation and in the absence of feedback the backward scattering on the Q12(1) transition was accompanied by complete suppression of the forward (concurrent) scattering.

Accurate electrical measurements for in situ diagnosis of RF discharges in plasma CVD processes

The electrical characterisation of radio-frequency (rf) discharges lead to a better understanding of the plasma processes, since their electrical properties are directly related to microscopic discharge properties. In this paper, we present a new method to obtain, simultaneously, discharge complex impedance, dissipated rf power and self-DC voltage, using a non-perturbative system for the electrical measurements and a rapid, self-consistent mathematical algorithm. The measurement system provides two operation modes: (a) a real time mode, for monitoring the discharge electrical parameters during a plasma process and (b) a fast acquisition mode, with a time resolution to the microsecond, to study the time evolution of the discharge electrical parameters during transient phenomena. Results for rf discharges in Ar and in SiH 4 are presented. The electrical measurements were obtained from a wide range of gas pressures (1–100 Pa) and rf powers (10–300 W). The evolution of discharge complex impedance and dissipated rf power are discussed in terms of their relation to discharge physical phenomena.

Surface temperature and thermal balance of probes immersed in high density plasma

The surface temperatures of thermal probes immersed in a low pressure inductively coupled argon discharge have been measured over a wide range of gas pressure. At a fixed discharge power, the measured temperature increases with gas pressure and decreases with increasing probe diameter. At a discharge power of 100 W, the surface temperature of a 0.4 mm diameter probe in the centre of the discharge ranges from C at 0.3 mTorr to C at 1 Torr. This temperature is considerably higher than the gas temperature. An analysis of the energy balance on a probe surface shows that plasma particle bombardment is the dominant heating process while radiation is the dominant cooling process. Probe temperatures found from an energy balance are in reasonable agreement with those measured in experiment.

Forward scattering of resonance laser radiation in He

The forward scattering of a laser beam, resonant to the 23P-33D He transition, is investigated. A forward scattered signal is detected for a wide range of gas pressure - 0.3–6 Torr. The peculiarities of the signal due to the broadband excitation and the fine structure of the transition are discussed and compared with theory.

Experimental study of bi-directional pumping of a far-infrared laser

We show that bi-directional pumping can increase significantly the output power of optically pumped gas lasers. We demonstrate this effect in experiments with a methanol laser pumped by a CO 2 laser in a wide range of gas pressure for two different wavelengths, 119 μm and 170 μm. In our experimental conditions we obtain an enhancement of the maximal output power up to 70% as compared with uni-directional pumping of the same total intensity.

A rectangular large ECR plasma source

Large-area ECR processing plasmas have been developed. Magnetic fields are composed of permanent magnets located along closed lines, inverting the polarity line by line. At first a new type of microwave launching system which consists of a dielectric plate and metal bars was tested, by using a discharge chamber of circular geometry in a diameter of 30 cm. It was verified that the launcher was useful in the production of a large-area plasma in a wide range of gas pressures. By using this type of launcher, a plasma in a rectangular geometry with dimensions of 40 × 50 cm was generated. Basic data which lead to the solution used to obtain a large rectangular uniform plasma (uniformity better than ±5%) were obtained.

Pulsed discharges produced by high-power surface waves

The mechanisms of the ionization front advance in surface-wave-produced discharges are investigated using two experimental set-ups. The high-power surface waves are excited in a 3 cm wavelength band by a surfaguide and a novel type of launcher (an E-plane junction). The ionization front velocity of the surface wave is measured for a wide range of gas pressures, incident microwave power and initial pre-ionization. The experimental results are compared with theoretical ones based on three different models. The comparison between theory and experiment allows one to suggest a new interpretation of the ionization front's advance. The ionization front velocity is determined by a breakdown wave or an ionization wave in the electric field of a high-power surface wave in the zone near the ionization front.

Study of diamond thin-film growth mechanism in a filament-assisted excimer laser ablation system

Diamond and amorphous carbon films have been deposited on silicon (100) substrates by filament-assisted XeCl excimer laser ablation of graphite targets. The influence of process parameters on the growth mechanism of diamond were studied by depositing films over a wide range of gas pressures and substrate temperatures. The surface morphology and bonding of the deposited films were characterized by scanning electron microscopy, and Raman and electron-energy-loss spectroscopy. In these experiments, microcrystalline diamond films with growth rates comparable to conventional chemical-vapor-deposition and plasma-enhanced chemical-vapor-deposition techniques could only be obtained at substrate temperatures and hydrogen pressures greater than 700 °C and 1.3 mbar, respectively. Conversely, the best conditions for growing carbon films with predominately sp3-type bonding structures were at temperatures and pressures less than 300 °C and 0.3 mbar, respectively. These results suggest that carbon-hydrogen gas phase reactions as well as gas-surface reactions are both necessary for the formation of diamond.

Production of ions in open-ended region of coaxial-type microwave cavity

Characteristics of a microwave planar magnetron ion source are described. This source, using a tuned coaxial-type cavity which excited the plasma at the end of the coaxial line, is able to produce a 100-mm-diameter by 20-mm-thick plasma disk that is well matched, stable, and allows continuous operation over a wide range of gas pressures. The isotropic microwave propagation from the surface of the magnetized plasma column to the center is one of the most effective coupling methods to obtain a high-density disk-like plasma. Argon ion current densities of 20–50 μA/cm2 have been obtained at energies ranging from a few to 70 eV. The beam uniformity is ±2% within a diameter of 5 cm. This new source has the potential to produce low-energy broad-beams.

Breakdown voltage characteristic of a pseudospark device

The breakdown voltage of a pseudospark device is measured for a wide range of gas pressure and anode–cathode gap distance. The data are analyzed by least-squares-fit methods, and the breakdown voltage is expressed by an empirical function of the gas pressure and the anode–cathode gap distance. It is found that the breakdown voltage is a function of p2d, the product of the pressure squared and anode–cathode gap distance. This is in contrast to that of a parallel-plate system that is described by Paschen’s curve, where the breakdown voltage is a function of pd.

Development of coaxial ECR plasma source for tube inner coating

We developed an electron cyclotron resonance (ECR) plasma source with a coaxial microwave mode which has no cut-off length for coating inside a metallic tube of small diameter and shorter than the cut-off length for the microwaves used. In a discharge with a narrow gap less than the wavelength of the microwaves, both the magnetic field and the electron loss to the boundary play an important role in the condition for breakdown. We theoretically and experimentally investigated the gaseous breakdown conditions for microwave power in the presence of an external crossed magnetic field. Using a new calculation method to treat the effects of electron collisions in a wide range of gas pressures the result indicates that the ECR condition ( ω = ω ce where ω is the microwave frequency and ω ce is the electron cyclotron frequency) has significant advantages for such a narrow-gap microwave discharge. The plasma is found to be produced at B of around 875 Gauss for the pressure range from 1 × 10 −4 to 5 × 10 −3 Torr. The discharge area broadened with increasing gas pressure. Good agreement is found between the calculated and measured discharge area. The typical electron density n e and electron temperature T e were also measured to be 8 × 10 10 cm −3 and 10 eV, respectively, which parameters are suitable for reactive sputtering or plasma chemical vapour deposition.

Probe measurements of the space potential in a radio frequency discharge

The dc and radio frequency (rf) components (and harmonics) of the probe potential have been measured in the midplane of a 13.56-MHz parallel-plane rf discharge in argon over a wide range of discharge voltages and at gas pressures between 10 mTorr and 1.0 Torr. rf potential measurements were made with different input capacitances to determine the true magnitude of the rf plasma potential and the probe capacitance. For a symmetrically driven rf discharge, the rf plasma potential Vrf is mainly composed of the second harmonic of the driving voltage Vdr over a wide range of gas pressures. For high values of Vdr, Vrf ≊0.1 Vdr while the dc probe potential Vdc is about 0.4 Vdr. These results are in good agreement with corresponding theoretical predictions found in the literature. For an asymmetrically driven rf discharge with equal electrode area, the rf plasma potential has an additional fundamental harmonic component equal to half the rf driving voltage. Values of rf plasma potential and probe capacitance given here allow us to specify the requirements on probe circuitry for different kinds of probe measurements in rf discharges.

An experimental system for symmetric capacitive rf discharge studies

An experimental system has been designed and built to comprehensively study the electrical and plasma characteristics in symmetric capacitively coupled rf discharges at low gas pressures. Descriptions of the system concept, the discharge chamber, the vacuum-gas control system, and the rf matching and electrical measurement system are presented together with some results of electrical measurements carried out in an argon discharge at 13.56 MHz. The system has been specifically designed to facilitate external discharge parameter measurements and probe measurements and to be compatible with a wide variety of other diagnostics. External electrical measurements and probe measurements within the discharge show that it is an ideal vehicle to study low-pressure rf discharge physics. Measurements from this system should be comparable to one-dimensional rf symmetric capacitive discharge theories and may help to verify them. Although only a few results are given here, the system has been operated reliably over a wide range of gas pressures and should give reproducible and accurate results for discharge electrical characteristics and plasma parameters over a wide range of driving frequency and gas components.

Modeling of a plasma column produced and sustained by a traveling electromagnetic surface wave

We present an improved model of a plasma column produced and sustained by a traveling azimuthally symmetric electromagnetic surface wave. The axial density profile of the column as well as the wave dispersion and field and power characteristics are now specified by five parameters: (i) the collision frequency for momentum transfer ν, ν<ω (ω being the wave angular frequency), (ii) the number σ=ωR/c (R is the plasma column radius, c the speed of light), (iii) the number β indicating the gas-discharge regime (B=0 for the diffusion regime and 0<β≤2 for the bulk recombination one), (iv) the permittivity of the container (glass tube) εd, and (v) the number γ=1+d/R (d denoting the thickness of the tube), the last two parameters having been neglected earlier. The influence of εd and γ on the wave dispersion properties and the axial profiles of the plasma density, wave power, and wave electric field components has been studied numerically. Our improved model is tested with the experimental data [A. Sola, A. Gamero, J. Cotrino, and V. Colomer, J. Appl. Phys. 64, 3419 (1988)] of argon plasmas produced by high-frequency surface waves in three different tubes and in a wide range of gas pressures. The agreement between the theory and experiment is very good. The model has been also applied to another particular experimental setup and the results are compared with the corresponding theoretical curves obtained by Sá and Ferreira (Europhysics Conference Abstracts ESCAMPIG 88, Vol. 12H, p. 191) and with the experimental data of Saada, Bloyet, Dervisevic, and Laporte [Université Paris Sud, Rapport L.P. 220 (1983)].

A study of electron swarms in nitrogen gas by the voltage transient technique

The voltage transient technique has traditionally been limited to high-pressure (p>500 Torr) discharges so that electron diffusion can be neglected. In much previous work image terms have also been omitted from the analysis of the experimental data. An analysis of the voltage transient has been developed which includes all image terms and both electron and ion diffusion. This analysis is applied to data in nitrogen gas, and the results have been compared with previous values obtained by other techniques. It is established that the measurement of voltage transients can be used to obtain electron transport parameters over a wide range of gas pressure and of E/N so long as the present full analysis is used.

Kr*2 excimer emission from multi-atmosphere discharges in Kr, Kr-He and Kr-Ne mixtures

The authors present details of the design and performance of a UV pre-ionised transverse discharge device designed for operation at high gas pressures. Stable, arc-free glow discharges could be obtained in pulses of 50-200 ns duration in pure Kr at pressures up to 1.5 bar, and in Kr-Ne mixtures up to 10 bar. Strong emission in the VUV excimer band of Kr*2 in the region 130-160 nm was observed over a wide range of gas pressures, the intensity increasing approximately as the square of the pressure in pure Kr. Attempts to obtain laser action have not been successful to date.

Dynamics of light-induced separation of gases

An investigation was made of the time dependence of light-induced separation of a mixture of isotopic modifications of 13CH3F–12CH3F molecules formed as a result of vibrational excitation of CH3F molecules by CO2 laser radiation. Measurements were made in a wide range of gas pressures. The experimental results are compared with the theory of the effect.

Pressure-dependent transient behaviour of the population of atomic and molecular metastables in He discharge

The transient behaviour of the population of excited state helium atoms (2 3S 1, 2 1S 0, 2 3P) and molecules (2sσ 3Σ + u) under a fast pulsed discharge excitation in pure helium has been studied experimentally using the optical interferometric method and the absorption method with nanosecond time-resolution. Experimental results are presented of the time dependent behaviour of these populations in the afterglow over a wide range of gas pressure from 35 Torr to 3 atm.

Axial particle density gradient in direct-current discharges

Particle density gradients in the positive column of a dc discharge have been reported previously. An increase of the density at the cathode end of the discharge tube is due to the neutralization of ions at the cathode (cataphoresis), while a density increase at the anode end is caused by an imbalance of the momenta transferred by the electrons and ions to the neutral particles (electrophoresis). A general transport theory is presented which incorporates both effects. An approximate analytical solution shows that for a monoatomic gas, cataphoresis dominates electrophoresis when the ion mean free path for momentum transfer exceeds 2R/3 (R is tube radius). Numerical results are given for helium over a wide range of gas pressure.

Plasma Electron Energy Distribution of a Beam-Plasma Type Ion Source

The plasma electron energy distributions of a beam-plasma type ion source are experimentally investigated in relation to the ion source operational parameters. These parameters are the primary electron beam current (beam-plasma discharge current) and the pressure of the working gas. Using the experimental results, we show that the beam-plasma discharge is an effective method of heating a plasma, with the plasma electrons being distributed in the range of a few eV to a few tens eV. As a result, a plasma is produced with a density of more than 1011 particles/cm3. Beam-plasma type ion sources using the beam-plasma discharge are highly stable and controllable, and can be operated over a wide range of gas pressures and primary electron beam currents.