Volume 92, Issue 5, 01 September 2002
Index of content:
- PLASMAS AND ELECTRICAL DISCHARGES (PACS 51-52)
Influence of the thermodynamic equilibrium state in the excitation of samples by a plasma at atmospheric pressure92(2002); http://dx.doi.org/10.1063/1.1492869View Description Hide Description
The microwave induced plasmas have been successfully used as an excitation source in atomic emission and mass spectrometry for the analytical determination of substances. In this work a study of the influence of the thermodynamic equilibrium state over the capacity of sample excitation of an argon plasmaflame sustained by a surface wave at atmospheric pressure is presented. The state of the thermodynamic equilibrium in the discharge is determined by the relation between its temperatures and densities. The values of these parameters depend on the energy available in the discharge, which is also responsible for the excitation of the samples introduced into the plasma. We have compared the behavior of two characteristic parameters of plasma (electron density and temperature) and of the ArI level population with the microwave power. The results have shown that the values of these parameters and populations had a tendency to remain constant for microwave powers above a certain value. Thus, from 100 W only a part of the energy injected into the discharge is absorbed in the plasma and the plasma equilibrium state is not consequently modified. This behavior is the same as that found for atomic lines of both halogens and iron introduced as samples into the plasma and seems to show that if the plasma is close to thermodynamic equilibrium the excitation of the samples is favored.
92(2002); http://dx.doi.org/10.1063/1.1497457View Description Hide Description
The methane decomposition and the formation of hydrocarbons, in particular acetylene, in a microwave plasma were studied. It was found that pulsing the discharge presents major advantages over the cw operation. The effect of the operating parameters, including pressure (15–65 mbar), flow rate (33–190 sccm), and discharge power (16–81 W) was investigated, with the aim to improve the efficiency for methane conversion and to reduce the energy requirement for the formation of acetylene. Maximum values of the methane conversion degree over 90% were obtained. As a function of the discharge conditions, acetylene can become the main reaction product, with 80% selectivity. The minimum energy requirement for methane conversion was approximately 7 eV/molecule and for acetylene formation 20 eV/molecule. The results show that active species generated in the plasma contribute to the methanedissociation and influence the product distribution. The correlation between the dehydrogenation and the gas temperature supports the view of thermally activated neutral–neutral reactions.
Effects of magnetic field on pulse wave forms in plasma immersion ion implantation in a radio-frequency, inductively coupled plasma92(2002); http://dx.doi.org/10.1063/1.1499983View Description Hide Description
The time-dependent current wave forms measured using a pulse biased planar electrode in hydrogen radio-frequency (rf), inductively coupled plasma,plasma immersion ion implantation experiments are observed to vary in the presence of an external magnetic field Results further indicate that the magnitude of the pulse current is related to the strength and direction of the magnetic field, rf power, and pressure, but the pulse current curves can be primarily correlated with The plasma discharges are enhanced in all cases due to magnetic confinement of the electrons, enlargement of the plasma generation volume, and increase in the rf power absorbing efficiency. The plasma density diagnosed by Langmuir probe diminishes in front of the sample chuck with whereas the plasma is confined nearby the sidewall of the vacuum chamber at high magnetic field. The high degree of plasma density nonuniformity at high in front of the sample chuck is not desirable for the processing of planar samples such as silicon wafers and must be compensated. The reduction in the plasma density and plasma density gradient in the sheath can be accounted for by the changes in the pulse current wave forms.
Modeling of a capacitively coupled radio-frequency methane plasma: Comparison between a one-dimensional and a two-dimensional fluid model92(2002); http://dx.doi.org/10.1063/1.1500789View Description Hide Description
A comparison is made between a one-dimensional (1D) and a two-dimensional (2D) self-consistent fluid model for a methane rf plasma, used for the deposition of diamond-like carbon layers. Both fluid models consider the same species (i.e., 20 in total; neutrals, radicals, ions, and electrons) and the same electron–neutral, ion–neutral, and neutral–neutral reactions. The reaction rate coefficients of the different electron–neutral reactions depend strongly on the average electron energy, and are obtained from the simplified Boltzmann equation. All simulations are limited to the alpha regime, hence secondary electrons are not taken into account. Whereas the 1D fluid model considers only the distance between the electrodes (axial direction), the 2D fluid model takes into account the axial as well as the radial directions (i.e., distance between the electrodes and the radius of the plasma reactor, respectively). The calculation results (species densities and species fluxes towards the electrodes) obtained with the 1D and 2D fluid model are in relatively good agreement. However, the 2D fluid model can give additional information on the fluxes towards the electrodes, as a function of electrode radius. It is found that the fluxes of the plasma species towards both electrodes show a nonuniform profile, as a function of electrode radius. This will have an effect on the uniformity of the deposited layer.
Spatial population distribution of laser ablation species determined by self-reversed emission line profile92(2002); http://dx.doi.org/10.1063/1.1500419View Description Hide Description
We propose a method for determining the spatial distribution of population densities for the species in laser-produced plasma. Our method relies on the parameter fittings of the experimentally observed self-reversed emission profiles to the model which is based on the calculation of one-dimensional radiative transfer. Employed parameters in the model represent spatial distribution of emitters, absorbers, and plasma free electrons. Since the density of plasmaelectrons has a spatial dependence, Stark shifts and broadenings are incorporated in a position-sensitive manner. After a general description of the method, we have specifically applied it to the laser-ablated Al plasma, where Al(I) emission line is employed for the analysis. In this specific example, we find that the accuracy of the fittings is significantly improved due to the presence of two emission lines originating from the fine structure, i. e., and In particular, the depth of the self-reversed structure turns out to be very sensitive to the position-dependent upper and lower level populations, which enables us to accurately determine the spatial variation of the laser-ablated species in these states. Furthermore, the calculated profile is almost unchanged with temperatures employed for fittings. This means that the present method gives reliable values of the parameters for the spatial distributions, even if the temperature is not precisely known.