René Erre

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 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.

Surface morphologies of composites based on unsaturated polyester pre-polymer

We report the study of surfaces of bulk molding compounds (BMC) based on miscible polymeric thermoset blends (TB)—unsaturated polyester, styrene and low profile additive (LPA)—containing fillers and glass fibers. In contrast to scanning electron microscopy (SEM) that identified a continuous organic layer at the BMC surface, atomic force microscopy (AFM) showed the existence of aggregates linked together to form a network at the micrometric scale. This indisputably demonstrated that phase separation took place at the surface of the BMC. The influence of TB was examined by comparing the surface morphologies of BMC and corresponding TB. Several cases were distinguished as a function of the TB composition. (1) Without LPA, the surface of the TB was continuous (no phase separation took place during curing) and the surface of BMC revealed the presence of aggregates resulting from a phase separation induced by the fillers. (2) For very low molecular weight LPA, aggregates randomly spread on islands surrounded by large holes were observed on the TB surface. These holes were shown to result from surface deformations induced by absence of shrinkage compensation. The corresponding BMC presented particles randomly spread on the surface. (3) The general case (higher molecular weight LPA) corresponded to similar TB and BMC surfaces morphologies with aggregates randomly spread on the surface. In this case, BMC roughness and morphology reflected the TB roughness and morphology. These observations led to the proposal of some considerations concerning the control of surface aspects of BMC.

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.