François Rouelle

Electrogeneration and some properties of the superoxide ion in aqueous solutions

Summary In the presence of surfactants such as quinoline which can form compact hydrophobic films, the inhibition of the polarographic reduction of oxygen selectively blocks the second electron transfer, and yields as main product the superoxide ion O 2 − . Under polarographic conditions, the reaction O 2 +e⇌O 2 − nearly achieves reversibility. By comparing the data obtained by pulse polarography and reoxidation chronocoulometry, the standard potential of the system has been determined ( E o =−0.270 V/NHE). From this value and the p K of the acid-base reactions between O 2 − , O 2 H and O 2 H 2 + , the full potential-pH diagram of the system O 2 o −O 2 −1 −O 2 −2 has been established. It shows that thermodynamic stability of O 2 − can only be expected in molten hydroxides—in agreement with fact. Electroreduction of O 2 − is a highly irreversible process. The non-additivity of the polarographic diffusion currents due to O 2 − generation and ferricyanide reduction, indicates that these two species react chemically in the diffusion layer. The behavior of electrodes other than mercury is briefly examined, in view of the possible role of O 2 − as first intermediate.

Electrochemical and Radiochemical Study of Copper Contamination Mechanism from HF Solutions onto Silicon Substrates

The mechanism of copper contamination of silicon wafers from dilute HF solutions containing ultratrace levels of metallic ion impurities, was investigated using a new electrochemical cell, which proved to act as a very efficient sensor for in situ characterization. Upon copper contamination, the open-circuit potential was observed to shift rapidly toward more positive values at a rate nearly proportional to the copper concentration. All potential/time curves tend to reach a plateau, while quantitative measurements using radioactive tracers revealed that during a few tens of minutes, copper ions were continuously reduced on the silicon surface. Results are interpreted in terms of the mixed-potential theory and lead to the conclusion that copper nuclei act as a catalyst which enhances the cathodic activity for proton reduction. The model was supported by atomic force microscopy observations which showed the initiation of corrosion pits around the nuclei.

p‐ and n‐Type Silicon Electrochemical Properties in Dilute Hydrofluoric Acid Solutions

After a systematic study of the factors influencing the electrochemical characteristics of the silicon/HF solution junction, we have obtained reproducible and reliable values of the electrochemical kinetic parameters of the interface. One of the features of this system is that the corrosion reaction. on anodic and cathodic sites is equivalent to two redox reactions, one at the energy level of the conduction band, the other at the level of the valence band. Then, we supported the assumption that the junction with Si can be treated by the electrochemical model. Data have been obtained using n- and p-type silicon with different doping levels, in contact with deoxygenated or oxygen-saturated 5% HF aqueous solution, in the dark and under illumination. The electrochemical reaction kinetics are expressed as a corrosion rate in atom cm -2 s -1 for different Si substrates.

Kinetics of electrochemical corrosion of silicon wafers in dilute HF solutions

Abstract An extensive experimental study of the factors influencing the electrochemical characteristics of the silicon/DHF junction has been undertaken, and leads to reproducible and reliable values of the electrochemical kinetics of the corrosion reactions. The usual model of electron and hole transfers between a semiconductor and an electrolyte solution should include an additional term due to the generation of h+ and e− charges resulting from the dual redox reactions on anodic and cathodic sites. Then, in a narrow range of potential near the corrosion conditions, the classical Butler-Volmer electrochemical equations apply. The values of open circuit voltage and corrosion current have been obtained using n- and p-type silicon with different doping levels, in contact with deoxygenated or oxygen-saturated DHF solution, in the dark and under illumination. These data were used to characterize the electrochemical reaction kinetics leading to the corrosion rate expressed in atoms per square centimeter per second of different Si substrates. In addition, we derived an estimation of the exchange current density of the hydrogen evolution reaction on the Si surface.

Corrosion Rate of n‐ and p‐Silicon Substrates in HF, HF + HCl, and HF + NH 4 F Aqueous Solutions

A research program was initiated in order to investigate the electrochemical corrosion of n- and p-type silicon substrates in 0.25 M dilute HF solutions, and the influence of fluoride ions or proton additives. All experiments were conducted in both the dark and under constant light flux, with solutions thoroughly degassed by high purity argon bubbling. Polarization resistance measurements near an open-circuit potential lead to the value of the corrosion current, while scanning the potential in the range of anodic and cathodic reactions permitted evaluation of the kinetics of charge transfer as a function of the majority carriers density in the semiconductor and the ionic composition of the solution. The influence of these parameters on the surface roughness of the silicon samples was also examined by ex situ atomic force microscopy profile measurements.

Electrochemical test for silicon surface contamination by copper traces in HF, HF+HCl and HF+NH4F dilute solutions

The electrochemical open circuit potential response described in a previous publication proved very efficient for the study of silicon wafer contamination by copper traces from HF solutions containing 20 to 800 ppb Cu2+ ions. In pure 0.5% DHF, copper nuclei were immediately generated at the silicon surface. In the same conditions, when the solutions contained 0.5% DHF+HCl 1 M, no electrochemical response was observed leading to the conclusion that silicon contamination was greatly inhibited. Upon NH4 F 1 M addition to the 0.5% DHF solution, the surface seems to be transiently contaminated and then tends to be partly cleaned. Further studies, of surface contamination, using radioactive 64 Cu as tracer, confirmed that HCl addition to HF solutions was efficient to generate an extremely passive silicon surface and supported the conclusions derived from the free potential measurements.

Copper Contamination Mechanism of Silicon Substrates from HF Solutions

The contamination of silicon wafers from dilute HF solutions containing ultratrace levels of metallic ion impurities is a subject of constant interest. The mechanism of copper electroless deposition from HF onto monocrystalline silicon was investigated using a new electrochemical cell, which proved to be a very sensitive detector for in situ characterization of silicon surfaces. Upon addition of copper trace amounts, the open-circuit potential was observed to shift rapidly towards more positive values at a rate nearly proportional to the copper concentration. All potential/ time curves tend to reach a limiting value of the potential, while quantitative measurements of radioactive tracers revealed that during a few tens of minutes, copper ions were continuously reduced on the silicon surface. Electrochemical potentials and voltammetric measurements were interpreted in terms of the mixed potential theory and led to the conclusion that copper nuclei act as a catalyst which enhances the cathodic activity for protons reduction. The model was supported by AFM observations which demonstrated the initiation of corrosion pits around the nuclei.