Caroline Boisse-Laporte

Generation and confinement of microwave gas-plasma in photonic dielectric microstructure.

We report on a self-guided microwave surface-wave induced generation of ~60 μm diameter and 6 cm-long column of argon-plasma confined in the core of a hollow-core photonic crystal fiber. At gas pressure of 1 mbar, the micro-confined plasma exhibits a stable transverse profile with a maximum gas-temperature as high as 1300 ± 200 K, and a wall-temperature as low as 500 K, and an electron density level of 10¹⁴ cm⁻³. The fiber guided fluorescence emission presents strong Ar⁺ spectral lines in the visible and near UV. Theory shows that the observed combination of relatively low wall-temperature and high ionisation rate in this strongly confined configuration is due to an unprecedentedly wide electrostatic space-charge field and the subsequent ion acceleration dominance in the plasma-to-gas power transfer.

Plasma-polymerized coatings using HMDSO precursor for iron protection

Abstract A large microwave plasma reactor was used for the deposition of plasma-polymerized coating from hexamethyldisiloxane (HMDSO)/oxygen (O 2 ) mixtures on iron samples. The plasma was characterized by optical emission spectroscopy as a function of the HMDSO to oxygen ratio. Infrared spectroscopy was used to investigate the coating structure. The variation of the HMDSO content in the feed provides a large wide of coatings, from silica (HMDSO content: 20%) to polymer (HMDSO content >80%). Electrochemical impedance spectroscopy was used to predict corrosion protective properties of the coating. Different HMDSO/O 2 ratios in the mixture provide varying degree of corrosion protection and the best protection was obtained for the plasma-polymerized coating from HMDSO 80% content precursor.

Hexamethyldisiloxane (HMDSO)-plasma-polymerised coatings as primer for iron corrosion protection: influence of RF bias

Silicone-like coatings were prepared by plasma enhanced chemical vapor deposition on steel substrates using a hexamethyldisiloxane–oxygen (80 ∶ 20) precursor mixture, in a microwave reactor. The plasma composition, the structure and barrier properties of the coatings were studied as a function of the RF bias. OES was carried out in order to identify the excited species in the plasma. The coatings were characterized by several ex situ diagnostics including FTIR, SEM, gravimetry and EIS. OES, using argon as an actinometer, showed no modification in the HMDSO dissociation as the RF bias varies; but it indicated changes in the chemical recombination reactions. Coating analyses revealed that radio frequency (RF) bias induced a densification of the coatings associated with modifications in their morphology and chemical composition. Results were explained using the general mechanism in the literature relative to plasma-deposited thin films. EIS results indicated that the best barrier properties during immersion in NaCl solutions were obtained with coatings deposited under high RF bias.

Silicon dioxide deposition in a microwave plasma reactor

Abstract SiO x C y H z films were deposited using hexamethyldisiloxane (HMDSO) in a 2.45 GHz microwave plasma reactor. The plasma was characterised by optical emission spectroscopy as a function of the HMDSO-to-oxygen ratio. The emission of species resulting from the dissociation and recombination of the monomer was identified. The film structure was investigated by means of FTIR spectroscopy and ellipsometry was used to determine the thickness and refractive index of the layer. The infrared spectrum and the deposition rate are strongly dependent on the HMDSO-to-O 2 ratio, whereas the refractive index is quite constant and lower than expected.

Microwave discharges produced by surface waves in argon gas

Reports on an experiment in argon discharges produced by surface waves at 2.45 GHz. A self-consistent description of these discharges at low pressure (p<10 Torr) is presented. Characteristics such as the electron density, the maintaining electric field (E), the collision frequency for momentum transfer v and the mean power required for maintaining the electron-ion pair theta are determined as functions of the product of the pressure and the plasma diameter. In particular scaling laws are verified for v and theta . It is concluded that experimental surface-wave discharges behave similarly to positive-column discharges.

Characterisation of a low-pressure oxygen discharge created by surface waves

Low-pressure oxygen discharges created by surface waves are characterised. Three discharge characteristics, the electron-neutral collision frequency nu , the maintaining electric field Eeff and the power required to maintain one electron theta , are deduced from the measurement of the electron density. They are independent of the microwave power and are studied as functions of the tube diameter (8-52 mm), the pressure (0.05-2 Torr) and the excitation frequency (390 or 210 MHz). The concentrations of ground state and singlet molecular oxygen are measured by VUV absorption and the concentration ratio of singlet to ground state molecular oxygen is about 10%, whatever the microwave power and the pressure. The ground state atomic oxygen concentration is measured downstream from the discharge by VUV absorption. The dissociation rate increases with the microwave power but remains less than 5%. A power balance model in the discharge, i.e. calculation of theta , is performed and yields a relation between theta /N and Eeff/N (n being the total neutral density), which is in good agreement with the experimental results. Finally, similarity laws are shown in the form theta /N and Eeff/N (Nr), where r denotes the tube radius.

Wave propagation and diagnostics in argon surface-wave discharges up to 100 Torr

Deals with the study of argon surface-wave produced discharges as the effective collision frequency nu becomes larger than the excitation pulsation omega (either at low frequency or at high pressure). Both conditions have been studied in the experiment reported. Discharges have been produced in the 1-100 Torr range in capillary tubes with 210 MHz and 2450 MHz excitation frequency. First, the set of Maxwell equations and boundary conditions is solved taking into account the attenuation due to collisions. Then the nu / omega influence is discussed. Secondly a diagnostic method providing the axial density profile ne(z) and the collision frequency nu is reported. Finally the authors find scaling laws for these discharges.

Influence of the radial electron density profile on the determination of the characteristics of surface-wave-produced discharges

Until recently, surface wave discharges have been studied, both experimentally and theoretically, using the wave propagation constant calculated in the assumption of a flat electron density profile. This paper deals with determination of wave characteristics (wavenumber, attenuation and fields) taking into account a radial profile of the electron density. The consequences for the experimental diagnostics are presented. In the range of low electron densities, the authors show that some results deduced from experimental measurements are spoilt if the radial electron density profile is not taken into account. They present an argon surface wave discharge (390 MHz) at low pressure (about 1 Torr) in a 76 mm diameter tube. The influence of the radial electron density profile is studied on the following experimental results: electron density, effective electron-neutral collision frequency and the mean power needed to maintain an electron in the discharge.

Spatial investigation of a large diameter microwave plasma

A large diameter plasma (120 mm) is sustained in argon/oxygen mixtures, at low pressure ( approximately=1 Torr) by surface waves at 2.45 GHz. We spatially describe the electromagnetic field distribution as well as the species distribution from diagnostics such as electric antenna probing and emission spectroscopy in the three spatial directions (z, r, phi ). According to azimuthal measures ( phi ), we show that the discharge is mainly maintained by an hexapolar hybrid mode (m=3). Longitudinal measures (z) exhibit modulations which could be explained by a multimode propagation whereas the radial spectroscopic measures (r) strongly depend on the electric field of the surface wave. However, atomic oxygen densities, nearly 1%, exhibit a quite homogeneous profile in the three spatial directions.

Experimental study of a pre-ionized high power pulsed magnetron discharge

This paper is focused on experimental studies of a high power pulsed magnetron discharge stabilized by low current pre-ionization. Time resolved studies were performed for a Cu target by optical emission spectroscopy and electrical measurements for different pressures of Ar buffer gas. Due to the elimination of the statistical delay time and a fast discharge current rise the quasi-stationary state was reached in 6 µs. The quasi-stationary state is characterized by an extremely high and pressure independent discharge current density of ~10 A cm−2 and stable Cu+ and Cu++ emissions. Such fast discharge dynamics permits the magnetron cathode current to be driven with a pulse of duration of the order of a few µs, significantly shorter than in other devices. During this short time, the plasma does not have time to undergo the transition from the glow to the arc discharge even at the extremely high cathode loads met in our case. Different stages of the fast discharge development are identified and the composition of the magnetized plasma as a function of the pressure is discussed in detail.