Layered Al₂O₃-SiO₂ and Al₂O₃-Ta₂O₅ thin-film composites for high dielectric strength, deposited by pulsed direct current and radio frequency magnetron sputtering

Multilayer thin films have the potential to act as high dielectric strength insulation for wire and microelectronics. In this study, films consisting of 2, 4 or 8 layers, composed of Al₂O₃ with SiO₂ or Ta₂O₅, were prepared via pulsed direct current and radio frequency magnetron sputtering to a thickness of between 152 and 236 nm. The dielectric strengths of all films exceeded the 310 Vμm⁻¹ achieved for PDC Al₂O₃. Maximum dielectric strengths were obtained for four layer composites; Al₂O₃-SiO₂-Al₂O₃-SiO₂ (466 Vμm⁻¹) and Al₂O₃-Ta₂O₅-Al₂O₃-Ta₂O₅ (513 Vμm⁻¹), each containing two PDC-Al₂O₃ and two RF-SiO₂/Ta₂O₅ layers. Whilst the average dielectric strength was higher in the Ta₂O₅ composites, they suffered from higher leakage prior to breakdown with ca. 6.5 nA compared to ca. 0.1 nA for SiO₂ composites. The mechanical properties of the composites were poorer due to increased intrinsic coating stress. Samples exhibited complete interfacial delamination with maximum coating adhesion strengths of 22 and 25 MPa. The variance resulted from larger coefficient of thermal expansion for Ta₂O₅ compared to SiO₂. Sputtered composites of Al₂O₃ and either SiO₂ or Ta₂O₅ had high breakdown strength with reasonable adhesion and could be suitable for insulating copper conductors in the aerospace and automotive industries.