Journal of Porous Materials 7, 27–31 (2000) c  2000 Kluwer Academic Publishers. Manufactured in The Netherlands. X-Ray Reflectivity Study of Formation of Multilayer Porous Anodic Oxides of Silicon VITALI PARKHUTIK Technical University of Valencia, 46071 Valencia, Spain Y. CHU, H. YOU, Z. NAGY AND P.A. MONTANO Argonne National Laboratory, MSD, Argonne, IL, USA Abstract. Present work reports a study of the morphology of thin (50–600 ˚ A) porous oxides of silicon grown in a special regime of the oscillating anodic potential. X-ray reflectivity (in-situ and ex-situ) was applied to analyze the morphology of oxides. It has been established that there is a direct correlation between a number of oscillations of potential during the oxide growth and the structure of oxide: it has a multi-layer structure with the number of layers corresponding to the number of oscillations. The results are interpreted using the model of the porous structures formation which explains the oscillatory oxide formation kinetics in terms of alternating processes of oxide formation and dissolution. Keywords: X-ray reflectivity, anodic oxide, oscillations 1. Introduction While polarizing silicon wafers galvanostatically in strong inorganic acids at increased temperatures one can observe very interesting phenomenon of the oscil- lations of the anodic potential value [1]. The oscilla- tions are very regular and well-shaped at low current densities and in concentrated electrolytes. The nature of the oscillations still is not clear. There exist two major points of view. One is that the oscilla- tions are the result of alternating processes of the ano- dic oxide growth and its local spontaneous dissolution [1, 2] similarly to what is suggested to other existing cases of oscillatory elecrochemical kinetics [3]. Other point of view is that the oscillations are not related with the porous anodic oxide growth but are the result of some phase transitions in the barrier oxide of silicon [4]. To support this assumption x-ray reflectivity from the surface of the anodic silicon oxide were referred to [4]. In this paper we provide further insight into the mechanism of the oscillatory kinetics of the anodic oxide growth on silicon. We have performed in-situ x-ray reflectivity measurements from silicon surfaces undergoing the reaction of electrochemical anodization and ex-situ measurements from the surfaces of series of the samples corresponding to different stages of the oscillatory anodic kinetics. 2. Experimental Set-Up and Results p-Si(100) wafers (2–6 ohm·cm) were used in the work. The choice of the conductivity type and doping level was not critical in the present case as the reactive layer is not the silicon but rather its anodic oxide. 0.2–1 M phosphoric acid was used as an electrolyte. X-ray re- flectivity measurements were made at 12BM beamline, Advanced Photon Source, Argonne National Labora- tory. Unfocused monochromatic beam of x-rays with