Determination of the elemental composition of micrometric and submicrometric particles levitating in a low pressure Radio-Frequency plasma discharge using Laser-Induced Breakdown Spectroscopy

International audienceThe LIBS (Laser-Induced Breakdown Spectroscopy) technique has shown its potential in many fields of applications including that of aerosol analysis. The latter is usually carried out on the particle flow, thereby allowing quantitative detection in various experimental conditions such as ambient air analysis or exhaust stack monitoring, to name but a few. A possible alternative method for particle analysis has been experimented combining a low pressure RF (Radio-Frequency) plasma discharge with the LIBS technique. Such approach has two peculiar features in comparison to the usual LIBS analysis. First, the particles injected in the RF plasma discharge are trapped in levitation. Second, the analysis is performed at a reduced pressure of around 1 mbar. LIBS detection at such low pressure has this peculiarity that particle vaporization is assumed to be achieved through direct laser particle interaction whereas it is caused by laser-induced plasma ignited in the gas at atmospheric pressure. The use of such particle trap could allow improving particle sampling, making organic particle analysis possible (by using an inert gas for RF plasma ignition) and even (depending on the pressure) obtaining a better signal to noise ratio. Detection of the elements of nanoparticle agglomerates made following their injection in the RF discharge has demonstrated the feasibility of such approach. Future experiments are intended to explore its potentialities when tackling issues such as process control or ambient air monitoring

Gas-plasma compressional wave coupling by momentum transfer

Pressure disturbances in a gas-plasma mixed fluid will result in a hybrid response, with magnetosonic plasma waves coupled to acoustic waves in the neutral gas. In the analytical and numerical treatment presented here, we demonstrate the evolution of the total fluid medium response under a variety of conditions, with the gas-plasma linkage achieved by additional coupling terms in the momentum equations of each species. The significance of this treatment lies in the consideration of density perturbations in such fluids: there is no 'pure' mode response, only a collective one in which elements of the characteristics of each component are present. For example, an initially isotropic gas sound wave can trigger an anisotropic magnetic response in the plasma, with the character of each being blended in the global evolution. Hence sound waves do not remain wholly isotropic, and magnetic responses are less constrained by pure magnetoplasma dynamics

In situ infrared absorption spectroscopy of dusty plasmas

In situ, time-resolved Fourier transform infrared spectroscopy was used to study particulate formation in rf discharges in mixtures of silane, argon, and nitrogen. The spectra were taken at a maximum rate of 20 Hz. The discharge conditions were chosen such that previous calibrations of the time evolutions of particle size and density could be used. The measurements indicate that the onset of the solid-state vibrational absorptions of the SiH and SiH2 bands only takes place after the nucleation and coagulation phase have finished; it coincides with the previously predicted start of the deposition of amorphous hydrogenated silicon on the particles. The dissociation of the silane feed gas is found to be in the range of 30%, and its time development suggests that also the large-scale dissociation of silane only starts after the coagulation phase. This is in agreement with previously observed trends for the electron temperature. If silicon partilces are grown in the plasma, and the silane flow is stopped, the Si particles stay trapped in the glow. The infrared measurements, however, show that they almost completely oxidize: the SiH/SiH2 vibrations disappear and a strong SiO vibration appears. If nitrogen gas is allowed into the plasma, the SiO vibration is replaced by a SiN vibration. © 1996 American Vacuum Societ

Capacitively coupled radio-frequency N2 discharges at low pressures

Capacitively coupled radio-frequency discharges (ccrf) in nitrogen mixtures are frequently used for the processing, modification and functionalization of different kinds of materials. Although nitrogen plasmas have been studied for many years, and despite their growing interest in applications, there is only partial knowledge about ccrf nitrogen plasmas. This paper uses experiments and modelling to study ccrf discharges in pure nitrogen, at 13.56 MHz frequency, 0.1–1 mbar pressures and 2–30 W coupled powers [1]. Experiments performed on two similar (not twin) setups, existing in the LATMOS and the GREMI laboratories, include electrical and optical emission spectroscopy (OES) measurements. Electrical measurements give the rf-applied and the direct-current-self-bias voltages, the effective power coupled to the plasma and the average electron density. OES diagnostics measure the intensities of radiative transitions with the nitrogen second-positive and first-negative systems, and with the 811.5 nm atomic line of argon (present as an actinometer). In the particular case of non-equilibrium ccrf discharges in nitrogen, a self-consistent modeling strategy must account for the interplay between the transport of particles, in the presence of density gradients and the rf field, and their production/destruction due to kinetic mechanisms involving both electrons and heavy species. Simulations use a hybrid code that couples a two-dimensional timedependent fluid module [2], describing the dynamics of the charged particles (electrons and positive ions N2 + and N4 +), and a zero-dimensional kinetic module, describing the production and destruction of nitrogen (atomic and molecular) neutral species [3]. The coupling between these modules adopts the local mean energy approximation to define space–time-dependent electron parameters for the fluid module and to work out space–time-averaged rates for the kinetic module. The model gives general good predictions for the self-bias voltage and for the intensities of radiative transitions (both average and spatially resolved), underestimating the electron density by a factor of 3–4.Fundação para a Ciência e a Tecnologia (FCT

In situ infrared absorption spectroscopy of dusty plasmas

International audienceIn situ, time-resolved Fourier transform infrared spectroscopy was used to study particulate formation in rf discharges in mixtures of silane, argon, and nitrogen. The spectra were taken at a maximum rate of 20 Hz. The discharge conditions were chosen such that previous calibrations of the time evolutions of particle size and density could be used. The measurements indicate that the onset of the solid-state vibrational absorptions of the SiH and SiH 2 bands only takes place after the nucleation and coagulation phase have finished; it coincides with the previously predicted start of the deposition of amorphous hydrogenated silicon on the particles. The dissociation of the silane feed gas is found to be in the range of 30%, and its time development suggests that also the large-scale dissociation of silane only starts after the coagulation phase. This is in agreement with previously observed trends for the electron temperature. If silicon partilces are grown in the plasma, and the silane flow is stopped, the Si particles stay trapped in the glow. The infrared measurements, however, show that they almost completely oxidize: the SiH/SiH 2 vibrations disappear and a strong SiO vibration appears. If nitrogen gas is allowed into the plasma, the SiO vibration is replaced by a SiN vibration

Polymorphous Si thin films from radio frequency plasmas of SiH4 diluted in Ar: A study by transmission electron microscopy and Raman spectroscopy

In this study, we present a detailed structural characterization by means of transmission electron microscopy and Raman spectroscopy of polymorphous silicon (pm-Si:H) thin films deposited using radio-frequency dust-forming plasmas of SiH4 diluted in Ar. Square-wave modulation of the plasma and gas temperature was varied to obtain films with different nanostructures. Transmission electron microscopy and electron diffraction have shown the presence of Si crystallites of around 2 nm in the pm-Si:H films, which are related to the nanoparticles formed in the plasma gas phase coming from their different growth stages, named particle nucleation and coagulation. Raman scattering has proved the role of the film nanostructure in the crystallization process induced ¿in situ¿ by laser heating