The properties of tantalum oxide (Ta2O5) metal-insulator-metal (MIM) capacitors with Al/Ta/Cu/Ta bottom electrodes were investigated. An ultrathin Al film successfully suppresses oxygen diffusion in the Ta2O5 MIM capacitor with the Cu-based electrode. The electrical characteristics and reliability of Ta2O5 MIM capacitors are improved by addition of ultrathin Al films. Ta2O5 MIM capacitors have low leakage current density (1 nA/cm(2) at 1 MV/cm) and high breakdown field (5.2 MV/cm at 10(-6) A/cm(2)). The decrease in leakage current is attributed to the formation of a dense and uniform Al2O3 layer, which has self-protection property and stops further oxygen diffusion into the tantalum contact. The dominant conduction mechanism of leakage current is the Poole-Frenkel effect at electric fields above 1.5 MV/cm. (c) 2006 The Electrochemical Society.