Oxidation temperature dependence of the dc electrical conduction characteristics and dielectric strength of thin Ta₂O₅ films on silicon

Abstract

Tantalum pentoxide thin films (60-80 nm thick) on silicon were prepared by thermal oxidation at 430-675°C of electron-beam evaporated Ta. The tantalum layers had been deposited by electron-beam evaporation onto the Si substrates held at room temperature (RT) or heated to 150°C during Ta evaporation. The dc conduction properties and the dielectic strength of the Ta2O 5 films were studied employing Al/Ta2O5/Si capacitors. The smallest conductivity was found for Ta2O5 films formed from Ta deposited onto Si held at RT during evaporation and oxidized at 490°C for 1 h in dry O2. This minimum conductivity corresponds to a leakage current of 1×10-7 A/cm 2 at an applied field of 1 MV/cm (Al negative on p-type Si substrates). The dc conduction characteristics can be interpreted by assuming Poole-Frenkel conduction. For Ta2O5 formed from Ta which had been deposited onto Si substrates held at 150°C during evaporation, the conductivity is smallest for a film which had been oxidized at 430°C for 1 h (lowest oxidation temperature investigated). Ta2O5 films formed from Ta deposited onto Si substrates at RT exhibit lower conductivity than Ta2O5 films formed from Ta deposited onto heated (150°C) Si substrates for a given oxidation temperature. The presence of water in the oxidizing ambient was found to greatly deteriorate the leakage characteristics of the Ta2O5 films. The dielectric strength of the Ta2O5 film for which the lowest conductivity was observed (oxidized at 490°C) ranged from 2 to 4.5 MV/cm. A maximum in the breakdown distribution was found for a breakdown strength of 3-3.5 MV/cm.