Gilles Cartry

Atomic oxygen recombination on fused silica: modelling and comparison to low-temperature experiments (300 K)*

This work is devoted to the study of atomic oxygen recombination on a glass surface, mainly in connection with atomic sources development. In this paper we present a non-stationary model for atomic oxygen recombination on a fused silica surface. Kinetics equations for oxygen atoms, taking into account heterogeneous reactions between gaseous atoms and the surface (Eley-Rideal mechanisms), as well as homogeneous processes involving surface migration of adsorbed species (Langmuir-Hinshelwood mechanisms), are solved. Surface reaction coefficients are calculated, and the choice of numerical values for surface parameters is discussed. The solution to the equations is compared to our previous experiments concerning the influence of the surface state on atomic recombination. An estimation is made of surface reaction coefficient values.

Gas temperature gradients in a CF4 inductive discharge

The neutral gas temperature in a CF4 planar inductive discharge was measured with space and time resolution using laser-induced fluorescence of the CF radical with analysis of the rotationally resolved excitation spectra. Strong temperature gradients are observed and temperatures as high as 900 K are reached at the reactor center at 50 mTorr with a power density of 0.15 W/cm3. The temperature at the reactor center increases with both gas pressure and power, but is independent of the gas flow rate. A simple model based on the global thermal energy balance is proposed to explain these results. The fraction of the injected rf power consumed in gas heating varies from 4.4% to 42% under our conditions (5–50 mTorr, 250 W rf power). Axial temperature profiles were measured in the steady state and in the time afterglow. The typical temperature relaxation times are several hundreds of microseconds. A numerical two-dimensional, time-dependent thermal model is in good agreement with the results.

Hydrogen-graphite interaction: Experimental evidences of an adsorption barrier.

The interaction of H atoms having relatively low average kinetic energy (∼0.025 eV) with both perfectly clean and D-covered HOPG surfaces is investigated using high resolution electron energy loss spectroscopy. From this study we confirm, in a controlled fashion, the presence of the theoretically predicted adsorption barrier since no adsorption is detected for such H atoms on HOPG. Moreover, we demonstrate that the exposure of a D saturated HOPG surface to these H atoms results in the complete removal of adatoms, with no further adsorption despite the prediction of the adsorption barrier to vanish for H dimers in para configuration. Therefore, the recombinative abstraction mechanism which competes with the adsorption process is more efficient.

Atomic oxygen recombination on fused silica : experimental evidence of the surface state influence

The time post discharge of a low-pressure pulsed dc discharge in pure oxygen is used to investigate the atomic oxygen recombination on fused silica surface. With the intention of studying this recombination for different surface states, we perform before each pulsed experiment a wall treatment by means of dc discharges under different experimental conditions. Then, we monitor the decrease of the atomic oxygen in time post discharge by time resolved VUV resonant absorption spectroscopy. We have shown that it is possible to obtain for a given wall treatment, a pulse after pulse variation of this decrease. We have attributed this variation to a filling of the chemisorption sites. Finally, we have determined the surface reaction probability of atomic oxygen on fused silica surface and we have compared it to published values.

Selective and deep plasma etching of SiO2: Comparison between different fluorocarbon gases (CF4,C2F6,CHF3) mixed with CH4 or H2 and influence of the residence time

SiO2 is a well suited material for integrated optic applications and is also attractive for microelectromechanical system and micro-optical electromechanical system fabrication. Such optical components require deep oxide etching (several microns) and subsequent high selectivity with respect to the mask. In this article, we describe the influence of various process parameters (gas mixture, pressure, plasma power, and residence time) on the selective etching of SiO2 with respect to Si in inductively coupled plasma (ICP) fluorocarbon with the aim of finding the best compromise between high selectivity and high oxide etch rate. Oxide etch rate is improved by decreasing pressure or increasing source power within the acceptable process windows, respectively, 3–20 mTorr and 1000–2000 W, but the gain in selectivity is low (×1.5). Adding methane rather than more commonly usual hydrogen resulted in higher selectivity without significant decrease in the oxide etch rate. A relatively good correlation is found between the selectivity and the (C+H)/F ratio of the precursor molecule. However, we show that varying the hydrofluorocarbon mixture does not allow us to improve both oxide etch rate and selectivity. In this regard, the residence time is the most significant parameter: choosing the appropriate amount of methane mixed with C2F6, and decreasing tR leads to an improvement in both the selectivity (×7) and the oxide etch rate (×1.5). Finally, the influence of these parameters on pattern transfer is investigated.SiO2 is a well suited material for integrated optic applications and is also attractive for microelectromechanical system and micro-optical electromechanical system fabrication. Such optical components require deep oxide etching (several microns) and subsequent high selectivity with respect to the mask. In this article, we describe the influence of various process parameters (gas mixture, pressure, plasma power, and residence time) on the selective etching of SiO2 with respect to Si in inductively coupled plasma (ICP) fluorocarbon with the aim of finding the best compromise between high selectivity and high oxide etch rate. Oxide etch rate is improved by decreasing pressure or increasing source power within the acceptable process windows, respectively, 3–20 mTorr and 1000–2000 W, but the gain in selectivity is low (×1.5). Adding methane rather than more commonly usual hydrogen resulted in higher selectivity without significant decrease in the oxide etch rate. A relatively good correlation is found between...

Etching mechanisms of Si and SiO2 in inductively coupled fluorocarbon plasmas: Correlation between plasma species and surface etching

Etching mechanisms of silicon and silicon oxide in a fluorocarbon environment are studied in an ICP reactor. Optimization of the process for deep etching of SiO2 with a Si mask has been discussed in a previous article. In this article, adequate plasma conditions are chosen both (a) to allow separation of parametric variables and (b) to get appreciable variation of the different plasma and surface experimental results versus parameters. Hence, pressure, source power, ion energy, and subsequently ion flux are kept constant. The influences of the gas composition and the residence time (varying gas flow rate) are studied. We show that silicon etching depends both on the atomic fluorine concentration in the plasma and of the fluorocarbon blocking layer at the surface. We pay particular attention to the formation of the fluorocarbon overlayer. We establish that the thickness of this layer is linked to the plasma species through the (C+H)∕F ratio, calculated from the radical densities in the gas, taking into acc...

Etching behavior of Si-containing polymers as resist materials for bilayer lithography: The case of poly-dimethyl siloxane

This work is focused on the plasma development of siloxanes investigated as model Si-containing photoresist components that show a promise for bilayer lithography at 157 nm and other Next Generation Lithography technologies. In such lithography, the image is developed in the top photosensitive polymer and transferred to the (usually thick) organic underlayer by means of O2-based plasma etching. In this work particularly, the issue of line edge roughness (LER) induced by transfer etching and its reduction by means of plasma processing optimization is addressed. The experimental results reveal that low values of line-edge roughness are obtained in a high-density plasma reactor, if an F- but not O-containing etching first step is used in appropriate plasma conditions. The effect of different etching chemistries and processing conditions on imaging layer roughness formation is demonstrated with the aid of scanning electron microscopy images and image analysis for quantifying LER, and atomic force microscopy (...

Experimental study and modelling of a low-pressure N2-O2 time afterglow

The time afterglow of a pulsed discharge is used to investigate the neutral-particle kinetics in N2-O2 low-pressure mixtures. The pressure is in the range 0.5-2 Torr at 300 K and the mixture composition in the range 0-20% of oxygen. Time-resolved emission spectroscopy on N2(B), N2(C), NO(A) and NO(B) is employed to monitor energy transfers involving the metastable state N2(A). The influence of N(4S) and O(3P) atoms on the kinetics is accurately treated using absolute concentration measurements by time-resolved absorption spectroscopy in the VUV range. It is shown that by an appropriate choice of the discharge repetition rate, the vibrational excitation of N2(X) can be neglected. A chemical model, containing few unknown parameters, is developed in order to fit the experimental fluorescences. The NO(X) kinetics are investigated and its absolute concentration is deduced. Furthermore, it is shown that the N2(A) density is probably higher for pulsed discharges than for stationary low-pressure DC discharges.

R&D around a photoneutralizer-based NBI system (Siphore) in view of a DEMO Tokamak steady state fusion reactor

ince the signature of the ITER treaty in 2006, a new research programme targeting the emergence of a new generation of neutral beam (NB) system for the future fusion reactor (DEMO Tokamak) has been underway between several laboratories in Europe. The specifications required to operate a NB system on DEMO are very demanding: the system has to provide plasma heating, current drive and plasma control at a very high level of power (up to 150 MW) and energy (1 or 2 MeV), including high performances in term of wall-plug efficiency (η  >  60%), high availability and reliability. To this aim, a novel NB concept based on the photodetachment of the energetic negative ion beam is under study. The keystone of this new concept is the achievement of a photoneutralizer where a high power photon flux (~3 MW) generated within a Fabry–Perot cavity will overlap, cross and partially photodetach the intense negative ion beam accelerated at high energy (1 or 2 MeV). The aspect ratio of the beam-line (source, accelerator, etc) is specifically designed to maximize the overlap of the photon beam with the ion beam. It is shown that such a photoneutralized based NB system would have the capability to provide several tens of MW of D0 per beam line with a wall-plug efficiency higher than 60%. A feasibility study of the concept has been launched between different laboratories to address the different physics aspects, i.e. negative ion source, plasma modelling, ion accelerator simulation, photoneutralization and high voltage holding under vacuum. The paper describes the present status of the project and the main achievements of the developments in laboratories.

Etching mechanisms of Si and SiO2 in fluorocarbon ICP plasmas: analysis of the plasma by mass spectrometry, Langmuir probe and optical emission spectroscopy

In this paper, we analyse, by the use of different plasma diagnostics, appearance potential mass spectrometry (APMS), optical emission spectroscopy (OES) and Langmuir probe measurements, a commercialized ICP source devoted to the etching of SiO2 using a Si mask. First, the influence of the gas composition (C2F6 mixed with H2 or CH4) and the residence time (varying gas flow rate) on the etching rates and selectivity is studied to optimize the process. Second, in order to improve the understanding of the etching mechanisms, the plasma is characterized according to the previous discharge conditions. We point out the presence of plasma instability due to the electronegative character of the fluorocarbon gas used. To determine the ion flux (i) which is an essential parameter for oxide etching, Langmuir probe measurements have been associated with a plot of the bias power versus bias voltage (Pbias(Ei)). Absolute concentrations of CFx (x = 1?3), CH3 and CHF2 radicals have been determined by APMS and the atomic fluorine concentration has been sampled by OES using argon actinometry. The techniques employed for concentration determinations are largely discussed. Finally, we compare the evolutions of the etch rates and the evolutions of the different plasma species with experimental conditions.