J. Bougdira

Spectroellipsometric analysis of CHF3 plasma-polymerized fluorocarbon films

Optical and morphological properties of fluorocarbon films (CFx) deposited on Si substrates from CHF3 in a reactive ion etcher were studied by spectroscopic ellipsometry (SE). The study has led for 0.3–1.5μm thick films to a thickness-independent, roughnessless, apparently nongraded two-layer model that requires the Cauchy dispersion and the Gaussian absorption for the film refractive index and extinction coefficient, and to an interface mixture of CFx, Si, and voids. The Si native oxide removal was ascertained from films formed on SiO2-sputtered Si substrate. The sample SE results were further verified by x-ray diffraction, atomic force microscopy, Fourier transform infrared, x-ray photoelectron and Auger electron spectroscopies, and capacitance studies. The film microstructure may thus be considered isotropic and homogeneous resulting from a random addition of CFx fragments including cross-links along the film depth. The films have low-dielectric constant(k≈2.2) quality index of 1.38(615nm) and are noti...

Agglomeration processes in carbonaceous dusty plasmas, experiments and numerical simulations

This paper deals with carbon dust agglomeration in radio frequency acetylene/argon plasma. Two studies, an experimental and a numerical one, were carried out to model dust formation mechanisms. Firstly, in situ transmission spectroscopy of dust clouds in the visible range was performed in order to observe the main features of the agglomeration process of the produced carbonaceous dust. Secondly, numerical simulation tools dedicated to understanding the achieved experiments were developed. A first model was used for the discretization of the continuous population balance equations that characterize the dust agglomeration process. The second model is based on a Monte Carlo ray-tracing code coupled to a Mie theory calculation of dust absorption and scattering parameters. These two simulation tools were used together in order to numerically predict the light transmissivity through a dusty plasma and make comparisons with experiments.

Measurements on argon ion by tunable diode-laser induced fluorescence in a low magnetic field helicon configuration reactor

This paper deals with the characterization of a RF reactor with a helicon configuration by studying a pure argon plasma. Tunable diode-laser induced fluorescence (TD-LIF) is performed in the center of the diffusion chamber to determine relative Ar+ metastable density as a function of RF power and static magnetic fields. A diode laser tuned at λ0 = 668.429 nm excites argon ions from the 3d4F7/2 metastable state to the 4p4D5/2 state, which decays to the 4s4P3/2 state with 442.60 nm fluorescence emission. Visible camera imaging, optical emission spectroscopy (OES) and RF compensated Langmuir probe measurements are used as complementary diagnostics. The results show that under our conditions, it is possible to work in three different regimes: the pure inductive, the simple Trivelpiece–Gould (TG) mode and the resonant TG mode. Moreover, these three regimes are very sensitive to the experimental plasma parameters. The decrease in the TD-LIF signal under particular conditions is associated with an increase in electron density and in argon ion emission lines in the 400–500 nm range. This phenomenon is assumed to be the consequence of the destruction of metastable states by electronic collisions.

Characterization of energetic and thermalized sputtered atoms in pulsed plasma using time-resolved tunable diode-laser induced fluorescence

In this work, a time-resolved tunable diode-laser (DL) induced fluorescence (TR-TDLIF) method calibrated by absorption spectroscopy has been developed in order to determine atom and flux velocity distribution functions (AVDF and FVDF) of the energetic and the thermalized atoms in pulsed plasmas. The experimental set-up includes a low-frequency (∼3 Hz) and high spectral-resolution DL (∼0.005 pm), a fast rise-time pulse generator, and a high power impulse magnetron sputtering (HiPIMS) system. The induced TR-TDLIF signal is recorded every 0.5 μs with a digital oscilloscope of a second-long trace. The technique is illustrated with determining the AVDF and the FVDF of a metastable state of the sputtered neutral tungsten atoms in the HiPIMS post-discharge. Gaussian functions describing the population of the four W isotopes were used to fit the measured TR-TDLIF signal. These distribution functions provide insight into transition from the energetic to thermalized regimes from the discharge onset. This technique may be extended with appropriate DLs to probe any species with rapidly changing AVDF and FVDF in pulsed and strongly oscillating plasmas.

Argon Ion and Neutral Metastable Levels Destruction in a Magnetically Enhanced Inductively Coupled Plasma Reactor

This paper deals with pure argon plasma studied in a magnetically enhanced inductively coupled reactor. Laser induced fluorescence technique was performed with an optical parametric oscillator laser on the 3d 4 F7/2 and 3d 2 G9/2 Ar + metastable levels and on the (2P3/2 0 )4s 2 neutral metastable state to determine their relative densities. Langmuir probe was used as a complementary diagnostic to measure the electron energy distribution functions. When increasing the confinement magnetic field, the metastable state density collapses for all cited levels, by contrast with n e which is continuously growing. Calculations were carried out for neutral argon to explain this behaviour. The results show that the metastable population is depleted by electron-impact excitation and ionization, these loss processes becoming dominant compared to the metastable state creation term.

Carbon Dust Growth in a Radiofrequency Discharge

Plasma wall interactions studies are of primary importance for increasing the life time of the first wall in fusion devices. In ITER, the divertor target plates will receive on a small surface a significant part of the power during operation, and carbon materials will be used. Although carbon has several advantages than the materials used at other places of the plasma chamber (W and Be), they undergo chemical reactions with hydrogen and its isotopes used as fuel for the fusion reaction. Under ITER operating conditions, the high temperature of the wall will promote diffusion and recombination of atomic hydrogen, withholding the fuel. Moreover, carbon atoms produced by erosion may be deposited at other locations, causing further increase of the hydrogen inventory in the vessel, and encountering several subsequent major safety issues.In our experiment, carbon dust formation and growth are studied in a radiofrequency discharge. Dust particles sediment into the cathode sheath using carbon originating either fr...