S Iordanova

Plasma diagnostics by optical emission spectroscopy on argon and comparison with Thomson scattering

A novel optical emission spectroscopy (OES) technique for the determination of electron temperatures and densities in low-pressure argon discharges is compared with Thomson scattering (TS). The emission spectroscopy technique is based on the measurement of certain line ratios in argon and a collisional‐radiative model (CRM) including metastable transport. The investigations are carried out in a planar inductively coupled neutral loop discharge (NLD) over a wide range of pressures, p = 0.05Pa‐5Pa. This discharge is a weakly magnetized novel radio-frequency (rf) plasma source, proposed for plasma etching. The NLD is operated in pure argon at a frequency of f = 13.56MHz and powers varied between P = 1kW and 2kW. Both diagnostics, OES and TS, are applied in parallel. The electron energy distribution functions obtained by TS are clearly Maxwellian at low pressures but exhibit a certain enhancement of the energetic tail at higher pressures. Electron densities and temperatures obtained by both diagnostic techniques are compared. Further, absolute numbers of the metastable densities derived from the measurement by the CRM are compared with earlier measurements under similar conditions. Excellent agreement is found throughout if depletion of the neutral gas density by increasing gas temperature and electron pressure is included in the CRM. Electron pressure is the dominant depletion mechanism at gas pressures p 0.1Pa and rf powers P> 1kW. There, the electron pressure exceeds more than 3 times the neutral pressure and the ionization degree approaches 7% while at pressures p> 1Pa the degree of ionization is relatively low (<10 −3 ) and neutral gas depletion is dominated by gas heating. (Some figures in this article are in colour only in the electronic version)

On the two modes of operation of planar-coil-driven inductive discharges in hydrogen

Langmuir probe and laser photodetachment technique diagnostics as well as optical emission spectroscopy and phase-resolved optical emission spectroscopy are used to study a planar-coil-driven inductive discharge in hydrogen sustained in the first chamber of a two-chamber plasma source. The discharge is operated in the power range 50?400?W at 27?MHz. The gas-pressure range studied is p?=?20?60?mTorr. The results obtained over the first half of the discharge length, that is starting from the position of the coil, outline discharge maintenance in the two modes of the inductive discharge. The inductive mode sustained at a high rf power appears with rf power deposition by ring-shaped rf electric field intensity close to the coil and high electron density there, followed by its strong drop in the remote plasma region. The capacitive mode sustaining low-density plasmas also appears with two regions specified by different mechanisms of rf power deposition: plasma heating by an electron beam acceleration in the wall sheath during its expansion and Joule heating in the plasma bulk. The similarity?at high and low rf power?in the axial structure of the discharge over the second half of its length, that is touching the transition between the two chambers of the source, and the appearance of a second maximum of the dc potential there are related to the configuration of the source. The obtained axial variation of the negative ion density obeys that of the potential of the dc electric field in the discharge.

Hydrogen Degree of Dissociation in a Low Pressure Tandem Plasma Source

ABSTRACT Hydrogen dissociation degree in an inductively-driven tandem plasma source, operating at low pressure, is measured by applying optical actinometry method with an argon gas as actinometer. The gas temperature, a parameter in the investigations, is obtained from the rotational temperature, analyzing the intensity distribution of the Fulcher-α band. Several actinometric pairs are used for examining the dissociation degree and its pressure dependence. The influence−on the results−of the difference in the excitation cross sections for argon, commonly accepted, is analyzed. Two suitable actinometric pairs are proposed, one of which can also be applied for fast monitoring of the dissociation degree.

Spectroscopic study of neutral species in a planar-coil inductive discharge in hydrogen

This contribution reports on an experimental study of characteristics of the neutrals in a single element of a matrix source of negative hydrogen ions. The investigations are carried out in hydrogen discharges maintained at a frequency of 27 MHz, applied power varied in the limits P = (50–150) W and pressure in the range p = (90–160) mTorr. An optical emission spectroscopy diagnostics, based on analysis of the spectral line profile and the intensity distribution of the Fulcher-α molecular band, is used. The profile of the atomic line reveals the existence of thermal as well as of non-thermal (fast) hydrogen atoms in the discharge. For processing the experimental data a line shape model, accounting for details of the plasma kinetics and the fine structure of the line, is used. The temperature of the atoms in the excited n = 3 state, the mean energy of the fast atoms, the ratio between the densities of these two groups of atoms and the relative population of the fine structure components of the n = 3 hydrogen state are determined. The atomic temperature is estimated to be equal to the temperature of the excited (n = 3) atoms. The molecular temperature is determined from the analysis of the intensity distribution of the molecular Fulcher-α band and is found to be about two times lower than the atomic temperature.

Investigation of the hydrogen neutrals in a discharge source used for production of metal hydrides

The paper discusses the possible mechanisms for production of metal hydrides (MH) in a DC discharge source. The results of different experiments suggest that the molecules are sputtered directly from the surface of the cathode, where they are formed after adsorption of atomic hydrogen. This hypothesis allows one to understand the operation of the source studied and to optimize its working conditions.

Analysis of H.ALPHA. spectral line profile in a DC hydrogen discharge

This contribution reports an experimental study of characteristics of the neutrals in a DC hydrogen discharge, designed for production of NiH. The investigations are carried out at gas pressure in the range p = (200–1600) mTorr and current I = 200 mA. Optical emission spectroscopy diagnostics are based on analysis of the H α line profile. The profile is formed from the emission of three energetically distinct groups of atoms in the n = 3 excited state. The first one is the group of thermalized atoms, with a mean energy of about 0.1 eV, the second—fast atoms, with mean energies about 10 eV and the third—superfast atoms with a mean energy of about 90 eV. The experimental technique leads to accurate values of the relative densities of these atoms and their mean energies for various working conditions.

Microwave discharge in a finite length vessel

A microwave surface-wave discharge at low pressure in a finite length vessel is experimentally investigated. Argon plasma is created in a dielectric capillary with length of 15 mm, the tube being an extension of an open-ended coaxial structure. Microwave power at frequency 2.45 GHz is coupled into the source applicator at power levels 6-15 W. The plasma column increases with the power applied and standing wave mode is realized when its length is equal to the capillary length. The electron temperature and plasma density are obtained simultaneously by passive optical emission spectroscopy using the line-ratio method. The surface waves phase-diagram shows that their wavelengths are much lower than the free-space wavelength. The input impedance of the plasma column - modelled as a lossy transmission line - has resonance behaviour when its length is equal to λg/2 or λg (λg being the surface waves wavelength). The results obtained for the plasma parameters, the surface waves wavelength and the vessel length show correlation to the power reflected, absorbed and radiated from the plasma column. The radiation pattern of the column shows that the main lobe is nearly perpendicular to the capillary axis.

A collisional radiative model of hydrogen plasmas developed for diagnostic purposes of negative ion sources

A collisional radiative model of low-pressure hydrogen plasmas is elaborated and applied in optical emission spectroscopy diagnostics of a single element of a matrix source of negative hydrogen ions. The model accounts for the main processes determining both the population densities of the first ten states of the hydrogen atom and the densities of the positive hydrogen ions H(+), H2 (+), and H3 (+). In the calculations, the electron density and electron temperature are varied whereas the atomic and molecular temperatures are included as experimentally obtained external parameters. The ratio of the Hα to Hβ line intensities is calculated from the numerical results for the excited state population densities, obtained as a solution of the set of the steady-state rate balance equations. The comparison of measured and theoretically obtained ratios of line intensities yields the values of the electron density and temperature as well as of the degree of dissociation, i.e., of the parameters which have a crucial role for the volume production of the negative ions.