Valérie Bertagna

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.

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.

Electrochemical study for the characterisation of wet silicon oxide surfaces

Abstract Because wet ultra-thin silicon oxides are extensively used in the microelectronic industry, we have investigated the growth of these oxides in various aqueous solutions using three main electrochemical techniques: (i) open circuit potential variation with time; (ii) linear voltammetry in a narrow range of potential; and (iii) electrochemical impedance spectroscopy under various polarisation potentials, to collect quantitative data regarding the growth kinetics of silicon oxide passivating layer, mainly at room temperature (r.t.). In oxidising alkaline solutions, the surface silicon oxide layer reached a limiting thickness value with time, related to oxidation/dissolution stationary behaviour. This observation was confirmed using ellipsometry. It was possible to reach with electrochemical techniques and ellipsometry the etching rate of the silicon substrate under the oxide layer in alkaline solution. Another interesting observation in this study was that the oxide layer showed a pronounced permeability to ions and oxidising agents in alkaline media, while this phenomenon vanished in acidic solutions.

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.

R and C Impedance Components Equivalent to the Charge Distribution within Si-Substrate Depletion Layer

The zero current impedance of a silicon substrate in a semiconductor/oxide/electrolyte structure was used to identify the contribution of the depletion layer under various bias potentials. Careful measurements using p-Si in a HCl solution within the potential range of 0 to - 1 V vs. a saturated calomel electrode (SCE) led to the determination of the corresponding equivalent circuits as a function of the bias potential. Modeling the circuit as a constant phase element proved that the imaginary component was a pure capacitor C S C in parallel with a pure resistance R S C . Experimental data showed that these two components undergo a steep variation when the system approaches the silicon flatband potential situation. A novel fundamental development is presented, assuming that the gradient of potential inside the material is small enough for a simplified treatment based on the linearization of the exponential function. The steep increase in the vicinity of the flatband potential of the space charge capacitance and conductance was confirmed. This constitutes a useful tool for electrochemical studies to determine the flatband potential and band curvature as a function of the sample potential measured vs. the SCE reference electrode.

Neutron reflectivity study of ultrathin SiO2 on Si

Neutron reflectivity was applied to the study of ultrathin silicon oxide films, of interest due to the requirement for reduced dimensions of the elemental components in microelectronic devices [I. Eisele and W. Hansch, Thin Solid Films 369, 60 (2000); C. Battaglin et al., Thin Solid Films 351, 176 (1999)]. Silicon oxides were prepared using three different ways: Chemical, electrochemical, and thermal oxidation. From neutron reflectivity, it was possible to derive the oxide thickness, the Si/SiO2 interface roughness, and the density of the layer. In complementary measurements, the chemistry of the chemical and thermal surface layers was obtained by infrared spectroscopy. The anodic oxides were found to be as dense as thermal oxides, but the chemical one was less dense. This result was checked by Fourier transform infrared spectroscopy.

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.