DIELECTRIC-PROPERTIES OF ANODIC OXIDE-FILMS ON TANTALUM

The electrochemical behaviour of Ta-TaOx structures was investigated b y impedance spectroscopy. It was shown that, after the polishing treat ment, the electrode is covered by an ''initial'' oxide film containing tantalum monoxide, TaO, and that ''electrogenerated'' oxides are only composed of tantalum pentoxide, Ta2O5 (greater than or equal to 97 mo l%). However, the dielectric properties of the structures strongly dep end on the thickness of the oxide, d(ox), because of the influence of a sub-stoichiometric oxide of TaO in thin layers, in agreement with pr evious results obtained by XPS measurements. The variation of the reci procal total capacitance of the oxide, C-ox, with the quantity of elec tricity, Q(a), involved in the formation of the oxide exhibits two lin ear parts. The breakdown of the slope of that curve is interpreted by a variation of the relative permittivity, epsilon(r), of the oxide wit h its thickness: for thin films (d(ox) less than or equal to 19 nm), e psilon(r) approximate to 18.5 and for thick films (d(ox) > 19 nm) epsi lon(r) approximate to 27.5.