Nanostructured anodic-alumina-based dielectrics for high-frequency integral capacitors

摘要

Three types of thin solid films with the nanoscale inner structures were synthesized by sputtering-deposition and anodizing of Al layer, Al-1.5 at.% Si alloy layer, and Al/Ta bilayer on Si wafers. All the anodic films comprised 1 μm thick nanoporous alumina layer as the key component. The essential differences were due to the silicon impurities (AlSi alloy) and the array of nanosized tantalum oxide protrusions in the alumina barrier layer (Al/Ta bilayer). The films were examined by scanning and transmission electron microscopy and electrochemical impedance spectroscopy. Integral capacitors utilizing the anodic films as dielectrics combine the small-value capacitance (6.5 nF cm~(-2)) with the excellent properties of high withstand field strength (1.7-2.7 MV cm~(-1)), low leakage current ((3-20)×10~(-12) A mm~(-2) at 1.0 MV cm~(-1)), and low loss tangent (tgδ = (4-6) × 10~(-3)). The revealed dispersion of dielectric constant, although within 10%, and the presence of loss peaks on the temperature and frequency dependencies of tgfi denote the influence of ion-relaxation mechanism on dielectrics' polarizability, with the characteristic times ranging from 10 to 145 μs depending on the dielectric type. By selecting appropriate technological and electrolytic conditions, the functionality of the capacitors can be optimized to meet the needs of a specific range, from 1 kHz to about 300 MHz operating frequencies.