M. Chemla

Survey of the metal nucleation processes on silicon surfaces in fluoride solutions: From dilute HF to concentrated NH4F solutions

Abstract As is well known, contamination of the silicon surface by trace metal impurities is responsible for detrimental effects in the production of ULSI circuits. An extensive experimental study of the factors influencing the spontaneous metal nucleation from fluoride solutions on Si substrates was undertaken. In acidic media (dilute HF solution) only noble metals can be deposited. The mechanism for the formation of Cu element nuclei was chosen as a model example. The first stage was the appearance of Cu crystals of a nanoscopic scale, observed by AFM microscopy. These nuclei soon induce corrosion pits due to the formation of a short-circuited electrochemical cell. In concentrated NH 4 F solutions, the open circuit potential (ocp) of Si samples is highly negative and provides an efficient driving force for nucleation even for common metals like Fe. Our results show that the deposition of Fe is hardly observable when Fe only is present, but in the presence of Cu, a catalytic effect is observed leading to the co-deposition of Fe+Cu nuclei. In all cases surface defects on the Si substrate are generated by the corrosion pits.

Surface processes: Effect of ohmic polarization on potentiodynamic V/I curves

Abstract Potentiodynamic V/I relationships are established in the general case of the formation of low-conducting films under ohmic resistance control. The model takes into account both the intrinsic conductivity of the surface compound and the possible thickening of the layer by ionic transport through the material. It is shown that generally the magnitude of the current peak and the corresponding potential increase linearly with the square root of the potential sweep rate. Some examples of resolution of the differential equations are developed, leading to analytical solutions I = F(V) by means of implicit relations involving the degree of coverage. A more elaborated mechanism, including a consumption reaction of the passivating compound coupled with the electrochemical process, is also proposed. The results are applied to the case of the passivation of graphite anodes, which are covered by graphite fluoride in fluoride melts.

Ionic mobilities of monovalent cations in molten salt mixtures

Abstract For a better understanding of the mechanism of electrical conductivity of ionic melts, additive binary systems yield more heuristic information than pure melts do. Thus, mobility isotherms of monovalent cations with a common anion are discussed. The difference in external and internal mobilities is explained. The methods for determining transport numbers of individual ionic species in mixtures are briefly mentioned. The profiles of the isotherms of the internal mobilities of monovalent cations in common anion binary systems may be classified into two types. In most of the binary melts a crossing of the isotherms occurs. Interpretation of the ionic mobilities reversal is given based on molecular dynamics simulation as well as experimental data, which is a clue for elucidation of the mechanism of ionic transport in molten salts.

Electrochemical study of mass transfer in Li−Mg and Li−Mg−Al alloys

AbstractThe interdiffusion coefficients in Li-Mg alloys and Li-Mg-Al alloys were evaluated using transient techniques such as chronopotentiometry and chronoamperometry. Anodic or cathodic pulses were imposed on the alloy electrodes under galvanostatic and potentiostatic conditions.Taking into account charging of the double layer, ohmic drop, adsorption of diffusing species and electrolyte-electrode boundary shift, the diffusion coefficients of lithium in Li-Mg alloys (α-phase and β-phase) and in Li-Mg-Al alloys were estimated at around 420°C. In the case of Li-Mg α-phase alloys, the values of the diffusion coefficients,DLi, can be represented in a polynomial expansion of the composition of the alloy,XLi (mol%) as follows:n

Mass spectrometric analysis of thermal decomposition products of graphite fluorides and electrogenerated carbon-fluorine compounds

ln D_{Li} = - 19.850 - 0.4294X_{Li} + 0.0249X_{Li}^2

An improved electrochemical cell for the characterization of silicon/electrolyte interfaces

nnThe diffusion coefficients of lithium in Li-Mg (β-phase) alloys show extremely large values (≃10−6 cm2s−1) as also in the Li-Al β-phase alloys.

Influence of bubble adherence on the kinetics of the fluorine evolution process

Abstract The interdiffusion coefficients in Al + Li alloys are measured by the use of transient techniques such as chronopotentiometry and chronoamperometry. The imposed pulses are both anodic and cathodic under galvanostatic and potentiostatic conditions. Taking into account the charging of the double layer, the adsorption of the diffusing species, and the movement of the electrode/electrolyte interface, the diffusion coefficients of Li in the Al + Li alloys (α-phase and β-phase) are determined at 422°C to be of the following forms as a function of the composition X mol Li percent: The surface composition of the alloys is estimated from a polynomial expression. The present results are in good agreement with those reported in the literature and obtained by other methods.

Diffusion et effet de surface dans la chronopotentiometrie de l'ion cuivreux dans l'eutectique LiCl-KCl fondu

Abstract Graphite fluoride is generally described as a stable compound at temperatures up to 400°C. It is shown that thermal decomposition starts below 100°C, leading to the formation of several gaseous fluorocarbons and some other compounds, e.g., CO, CO2, SiF4, when the reaction proceeds in a quartz tube. Comparison of the spectra of several graphite fluorides with those of samples obtained from the surface layers of carbon electrodes used as anodes in molten 2HF—KF, confirms the hypothesis of the formation of a CFχ passivating surface film. Its stoichiometric ratio if estimated to be χ ≈ 1.1, for a carbon anode maintained for several hours at +6 V vs. a Pt—H2 electrode. The formation of graphite fluoride is responsible for the high anodic overvoltage and contributes to the disintegration of the carbon anode. Analysis of the anodic gases evolved from a laboratory fluorine cell shows that gaseous fluorocarbons are evolved at a potential lower than that of fluorine evolution, in agreement with thermodynamic calculations. These results explain why fluorine always contains trace amounts of fluorocarbons.