Anodisation du silicium : oscillations electrochimiques et porosification

The electrochemical oxidation, namely anodizing, is a surface treatment technique. For instance, it is used in the industry to form protective layers on metals such as aluminium, iron or zinc. If this technique is applied to silicon semiconductor it can lead to different types of surface treatments. Depending on the anodizing conditions, it is possible to electropolish the silicon, to form a thin layer of silicon oxide or to porosify directly the silicon by electrochemical etching. This diversity of effects makes this material interesting to study. This thesis deals with a specific phenomenon occurring during the anodic formation of silicon oxide : the electrochemical oscillations. These oscillations have been studied extensively over the past decades because they stand as a very intriguing enigma. This study of the oscillations is a fundamental research that aims to improve the understanding of this oscillatory phenomenon and to better understand the growth mechanism of anodic silicon oxide. In particular, we analyze the impact of electrostriction, a type of electro-mechanical coupling in dielectrics, on the mechanical aspects of the oscillations. This thesis also deals with two types of porosification occurring during the anodic oxidation of silicon : porous silicon and porous silica formation. We could find new ways to improve the diversity of these porous morphologies.L’oxydation électrochimique, ou anodisation, est une méthode de traitement de surface employée notamment dans l’industrie pour former des couches protectrices sur des métaux tels que l’aluminium, le fer ou encore le zinc. Appliquée au matériau semi-conducteur qu’est le silicium, l’anodisation peut aboutir à plusieurs types de traitements de surface différents. En fonction des conditions d’anodisation, il est possible d’obtenir un électropolissage du silicium, la formation d’une couche d’oxyde de silicium sur le silicium, ou la porosification directe du silicium par gravure électrochimique. C’est notamment cette diversité qui fait l’intérêt de l’étude du silicium. Cette thèse s’intéresse à un phénomène spécifique qui se produit durant la formation d’une couche de d’oxyde de silicium : les oscillations électrochimiques. Ces oscillations ont été abondamment étudiées durant les dernières décennies car elles constituent une énigme particulièrement intrigante. Notre étude de ces oscillations est une recherche à caractère fondamental qui fait avancer la compréhension de ce phénomène oscillatoire et permet de mieux comprendre le mécanisme de croissance de l’oxyde anodique de silicium. Notamment, nous analysons l’impact de l’électrostriction, un phénomène de couplage électro-mécanique dans l’oxyde, sur l’aspect mécanique des oscillations. Dans un deuxième temps, cette thèse traite de la porosification par voie électrochimique, que ce soit la porosification du silicium ou de l’oxyde de silicium. Le travail réalisé dans cette thèse permet d’augmenter la diversité des morphologies poreuses disponibles, tant pour le silicium poreux que pour l’oxyde de silicium poreux.(FSA - Sciences de l) -- UCL, 201

In-situ monitoring of electrochemical oscillations during the transition between dense and porous anodic silica formation

This study investigates the mechanism responsible for the electrochemical oscillations during silicon galvanostatic anodizing. Two oscillatory regimes of anodic silica formation in dilute fluoride electrolyte are monitored by combined in-situ curvature measurement and ellipsometry. At lower applied current density, a dense silica film is formed and the oscillations features are similar to those observed in fluoride-free electrolytes. At higher applied current density, a porous silica film is formed and the oscillations progressively reappear after a transition regime without oscillations. The disappearance and the reappearance of the oscillations are associated to variations of the degree of synchronization between the self-oscillating domains. In addition, the similarities between the oscillatory regimes and the persistence of sustained oscillations during the formation of thicker silica indicate that all oscillations arise from a same mechanism that survives the oxide accumulation at the silicon surface

What controls the pore spacing in porous anodic oxides?

In this paper, we use energy-based perturbation criteria to examine whether strain or electrostatic energy acts as a driving force for porosity initiation in anodic oxides. By doing so, we also succeeded to rationalise the dependence of pore spacing on anodising conditions. Our experimental approach consists of measuring in-situ the internal stress in anodic oxide films grown galvanostatically on aluminium in phosphoric acid, and to correlate these data with the measured pore spacing of the obtained porous films. Our results indicate that the possibility of a strain energy-induced surface instability is unlikely, as for this case the constitutive dependence of pore spacing on internal stress was not verified. Instead, the measured pore spacing, electric field and barrier oxide thickness obtained on our anodic alumina films indicate that electrostatic energy is the main driving force for pore initiation, as well as the factor controlling the pore spacing. Corroborative quantitative evidence for this novel electrostatic-based scaling law is provided by data compiled from the literature for a range of other anodic oxide systems, including nanoporous alumina and nanotubular titania films. Keywords: Anodic oxides, Porous alumina, Perturbation analysis, Internal stress, Surface energy, Electrostatic

Control of mesoporous silicon initiation by cathodic passivation

We demonstrate that cathodic polarization is an effective method for in-situ control of surface morphology in the fabrication of mesoporous silicon. The cathodic currents are applied on the silicon electrode before or during the anodic formation of mesoporous silicon. On the surface, the formation of a parasitic microporous layer is avoided, the pores diameter can be increased from 5 to 32 nm, and the pore density can be decreased from 1900 to 600 μm - 2. A simple growth model is proposed to explain the surface morphology evolution based on the hydrogen passivation of the silicon surfaces in the cathodic regime. © 2013 Elsevier B.V

In situ monitoring of electrostriction in anodic and thermal silicon dioxide thin films

Stresses due to electric fields in thermal and anodic silica thin layers can impact the devices using these films as dielectrics. Accurately quantifying the internal stress as a function of the electric field is thus of technological importance. In this work, electrostrictive stresses are monitored during cyclic polarization of silica thin films on silicon and during the growth of anodic silica. These are obtained by combining curvature and ellipsometry measurements in situ. In silica films grown by thermal oxidation of silicon, the electric field can generate either tensile or compressive stresses depending on its magnitude and on the silica polarization history. The electromechanical coupling in thermal silica is assumed to be controlled by a reversible change of the dipole organization. For anodic silica films, the stress generated by the electric field is tensile and varies linearly with the square of the electric field above 0.26 V2/nm2 under both cyclic polarization and oxidation conditions

Towards an understanding of electrochemical oscillations during anodic silica formation

Abstract numéro 2

Electromechanical coupling in anodic niobium oxide: Electric field-induced strain, internal stress, and dielectric response

Seemingly, contradictory results have been reported so far for electrostriction in anodic oxides. Furthermore, no definitive agreement could be obtained with theory. In this paper, in situ techniques are combined to elucidate electrostriction in anodic niobium oxide. The dependence of strain, internal stress, and dielectric constant on the electric field is measured by, respectively, spectroscopic ellipsometry, curvature, and impedance measurements. The through-thickness strain is tensile and proportional to the square of the electric field. The in-plane internal stress is compressive and proportional to the square of the electric field at low field values. The internal stress is predicted relatively well by the Maxwell stress because of the weak dependence of the dielectric constant on the volume change of the oxide. The dielectric constant decreases with the electric field, the dependence being quadratic. While the evolution of the strain and stress with the electric field can be ascribed to the dependence of the dielectric constant on strain, the dependence of the dielectric constant on the electric field contains an explicit strain and electric field dependence. A mechanism for the latter is proposed. © 2012 American Institute of Physics

Morphological and electrical instabilities during anodic oxide growth (Invited presentation)

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