N. Sadeghi

Oxygen and fluorine atom kinetics in electron cyclotron resonance plasmas by time‐resolved actinometry

The kinetics of O and F atoms in O2‐based plasmas has been studied by time‐resolved optical emission spectroscopy (actinometry) in modulated plasmas. The sticking coefficient αO of O atoms on the stainless‐steel reactor walls was 0.09±0.01 in O2 plasmas containing fluorine (added as either SF6 or F2), but was about 0.5 in a pure O2 plasma. This explains the significant increase in steady‐state O density as a few percent of fluorinated gas is added. The corresponding value for F atoms, αF, was 0.06±0.01, almost independent of conditions. The method also indicates the relative importance of the different electron‐impact‐induced mechanisms (direct excitation of ground‐state atoms and dissociative excitation of feedstock molecules) for the production of emitting atoms [O 3p3P (844 nm) and F 3s2P (703 nm)] in plasmas. These results show that the widely used (steady‐state) actinometry technique using 844‐nm emission from O 3p3P atoms is an unreliable measure of ground‐state [O] variations.

OXYGEN ATOM ACTINOMETRY REINVESTIGATED : COMPARISON WITH ABSOLUTE MEASUREMENTS BY RESONANCE ABSORPTION AT 130 NM

Resonance absorption at 130 nm was used to measure absolute oxygen atom concentrations in O2‐containing distributed electron cyclotron resonance plasmas. The dissociation fraction [O]/[O2] in pure O2 plasmas (1–6 mTorr) was in the range 0.01–0.06, but was significantly increased by the addition of SF6, N2 or Kr. At 2 mTorr total pressure a maximum [O]/[O2] of 0.3 was observed for 10% SF6 added. The results were compared to those obtained by optical emission actinometry measurements. The quantity I0 (844 nm)/IAr (750 nm) (with 0.1 mTorr Ar added) was poorly correlated with [O] but well correlated with [O2]. This suggests that, for dissociation fractions lower than 0.1, dissociative excitation, O2+e→O*(3pu20093P)+O, is the most important mechanism for the production of 844 nm emission.

Comparison of the Ar(3P2) and Ar(3P0) reactions with chlorine and fluorine containing molecules: Propensity for ion–core conservation

Optical pumping has been used to select Ar atoms in the metastable 3P2 or 3P0 state in a flowing afterglow reactor. The relative concentrations of the two metastable states were assigned from observation of the N2(C,v’) emission spectra. The isolated reactions of the Ar(3P2) and Ar(3P0) atoms with F2, NF3, Cl2, CCl4, PCl3, and SOCl2 were examined at 300 K by observation of ArCl* and ArF* formation. The total quenching rate constants for Ar(3P0) are slightly larger than for Ar(3P2). The Ar(3P2) atom reactions give only the B and C states of ArF* and ArCl*; the Ar(3P0) atom reactions give a mixture of B, C, and D states with B and D being favored. Thus, a propensity for conservation of the Ar+ ion–core configuration was found. The branching fraction for ArX* formation from Ar(3P0) with Cl2, SOCl2, F2, NF3 are similar, but those for PCl3 and CCl4 are smaller, relative to Ar(3P2). Improved rate constants for formation of individual N2(C,v’) levels from Ar(3P0) and (3P2) reacting with N2 at 300 K are given in ...

Absolute radical densities in etching plasmas determined by broad-band UV absorption spectroscopy

Broad-band UV absorption spectroscopy was used to determine radical densities in reactive gas plasmas generated in a 13.56 MHz capacitively coupled parallel plate reactor. Five radical species were detected: , CF, AlF, and . Absolute (line-integrated) densities were determined in and plasmas, as were the vibrational and rotational temperatures in the latter case. In plasmas the CF radical was also detected, along with the etch products AlF (from the Al powered electrode) and (when an Si substrate was present). The fraction that comprises of the total etch products was estimated. Finally, the dimer was detected in an plasma in the presence of an Si substrate. This simple technique allows absolute concentrations of many key reactive species to be determined in reactive plasmas, without the need to analyse the complex rotational spectra of these polyatomic molecules.

Kinetics of N2(A 3Σu+) molecules and ionization mechanisms in the afterglow of a flowing N2 microwave discharge

To gain an understanding of the processes responsible for the formation of the well-known short-lived afterglow (SLA) or pink afterglow of nitrogen, different diagnostic techniques are implemented in the afterglow of a 440xa0Pa microwave nitrogen discharge in a 3.8xa0cm diameter flow tube. Using the intracavity laser absorption spectroscopy technique, we measure the space-dependent absolute density of N2(Axa03Σu+; v = 0) metastable molecules, as well as their rotational temperature, which in fact corresponds to the gas temperature, Tg. The density of N2(Axa03Σu+) molecules is about 5×1017xa0m-3 at the end of the discharge zone (z = 4xa0cm). It then continuously decays by almost two orders of magnitude to reach a minimum around z = 12xa0cm before monotonically increasing to a secondary maximum of 5×1016xa0m-3 located around z = 19xa0cm. It then slowly decays at longer distances. The space-dependent N2(Bxa03Πg) fluorescence intensity evolves in exactly the same way. A simple kinetic model is developed and we conclude that metastable molecules are locally formed in the SLA and not carried to this region by the gas flow. Contributions to the creation of N2(Axa03Σu+) molecules from N-N atom recombination, as well as from high vibrational levels of the ground-state N2(X,xa0v) molecules are analysed. To account for the high density of N2(Axa03Σu+) molecules, despite the (2-3) ×1021xa0m-3 density of N atoms in the afterglow, we propose that N(2P) metastable atoms, resulting from the efficient quenching of N2(Axa03Σu+) molecules by N(4S) atoms, react immediately with vibrationally excited N2(Xxa01Σg+; v≥10) molecules to again form N2(Axa03Σu+) molecules. The absolute electron density in the SLA was also measured by microwave interferometry. Its axial dependence also shows a pronounced minimum at z = 12xa0cm before reaching a maximum of 6×1015xa0m-3 at z = 19xa0cm. The possible processes for this local ionization, in the absence of any electric field, could be binary collisions of the electronically excited molecules, and/or of the vibrationally excited ground-state molecules in v≥30 levels.

Electron beam pulses produced by helicon-wave excitation

The 443 nm Ar+ line emission intensity in a cylindrical argon magnetoplasma excited by a 13.56 MHz helicon antenna is found to be strongly modulated at the excitation frequency. The peak in optical emission propagates in the axial direction at a velocity corresponding to that of helicon waves launched by the antenna. The spatiotemporal modulation is consistent with pulses of electrons produced by acceleration under the antenna and subsequent entrainment of these electrons over half of a trapping period in the axial electric field of the helicon wave.

Quenching rate constants for reactions of Ar(4p′[1/2]0, 4p[1/2]0, 4p[3/2]2, and 4p[5/2]2) atoms with 22 reagent gases

The total quenching rate constants of argon atoms in the 4p′[1/2]0, 4p[1/2]0, 4p[3/2]2, and 4p[5/2]2 states (2p1, 2p5, 2p6, and 2p8, respectively, in the Paschen numbering system) by rare gases, H2, D2, N2, CO, NO, O2, F2, Cl2, CO2, NO2, CH4, C2H2, C2H4, C2H6, CF4, CHF3, and SF6 have been determined at room temperature. These four excited states of argon (energy 13.09–13.48 eV) were selectively prepared by two-photon excitation from the ground state using VUV (184–190 nm range) laser pulses. The total quenching rates were deduced from the pressure dependence of the decay times of the excited-state atoms, measured by observing their fluorescence emission intensities in the presence of added reagents. The quenching constants increase from values of ≅0.01×10−10u200acm3u200aatom−1u200as−1 for Ne, to ≅0.1×10−10u200acm3u200aatom−1u200as−1 for He and Ar, and to very large values, (5–15)×10−10u200acm3u200aatom−1u200as−1, for most polyatomic molecules, F2, Cl2, and O2. The quenching mechanisms of the Ar(4p,4p′) atoms are briefly discussed and compar...

Self-consistent kinetic model of the short-lived afterglow in flowing nitrogen

A detailed kinetic model for the flowing nitrogen microwave discharge and post-discharge is developed with the aim of gaining a deeper understanding into the processes responsible for the formation of the short-lived afterglow of nitrogen and for the enhancement of the concentration of N2(A 3 � + ) metastable, measured at approximately the same position in Sadeghi et al (2001 J. Phys. D: Appl. Phys. 34 1779). The present work shows that the peaks observed in the afterglow, for the density of molecules in radiative N2(B 3 � g) and N + (B 2 � + u ) and metastable N2(A 3 � + u ) states, can be explained as a result of a pumping-up phenomenon into the vibrational ladder produced by near-resonant V–V energy-exchange collisions, involving vibrationally excited molecules N2(X 1 � + g ,v ) in levels as high as v ∼ 35. The present predictions are shown to be in good agreement with the measured concentrations for N2(A 3 � + ) metastables and N( 4 S) atoms, and with the emission intensities of 1 + and 1 − system bands of N2.

Symmetries, propensity rules, and alternation intensity in the rotational spectrum of N2 (C 3Πu) excited by metastables Ar(3P0,2)

We have obtained high resolution spectra N2(Cu20093Πu→Bu20093Πg) afterglow fluorescence resulting from excitation transfer from state selected Ar metastable 3P0 or 3P2. This was achieved by selective depopulation of one metastable by optical pumping using a tunable c.w. dye laser. Relative probabilities of excitation of well defined rotational, spin, and parity molecular Λ doublet substates were obtained for reaction of 14N2 and isotopic 15N2 with Ar*(3P2) and Ar*(3P0). A semiclassical model is proposed and found in fairly good agreement with the observed relative yields. The physical bases of the model are justified and imply that (a) spin–orbit interactions are effective during the collision, (b) the plane of rotation of the excited N2 molecule is a plane of symmetry, (c) Λ doublet c states are preferentially populated in the excitation transfer process.

Influence of magnetic field and discharge voltage on the acceleration layer features in a Hall effect thruster

The axial velocity of singly charged xenon ion is determined by means of laser induced fluorescence spectroscopy at the exhaust of a PPS100-LM Hall effect thruster by analyzing the Doppler shifted spectral profile of the 834.72 nm Xe+ ion line. Measurements are carried out both inside and outside the thruster channel. Ion velocity distribution functions (VDF) are used to compute the ion accelerating potential. It is shown that such a potential can in fact be defined in several ways due to the overlap between the ionization and acceleration layers, which translates into broad VDF. The PPS100-LM thruster is operated under an applied voltage ranging from 100 to 300 V and a magnetic field produced by coils whose current is varied from 2.5 to 5.5 A. The broad range of working conditions allows one to determine the influence of these two parameters on the ion dynamics. Finally, the experimental results are used to confirm the outcomes of a 2D kinetic model.