Tantalum Oxide-Silicon Oxide Duplex Dielectric Thin-Film Capacitors

Tantalum oxide possesses several outstanding characteristics as a dielectric for thin-film capacitors: a relatively high dielectric strength, a high capacitance-to-area ratio, and good mechanical and electrical stability. The high capacitance density, however, becomes a disadvantage for capacitors in the range of about 100 pF and below. Silicon oxide has been widely investigated as a dielectric for low value capacitors, but it is limited by pinhole defects. By depositing SiO on Ta 2 O 5 to form a duplex dielectric structure, low value capacitors and large area distributed RC networks can be realized and incorporated into tantalum integrated circuitry. Because of the small probability of alignment between defects in the two dielectrics, high initial yields and improved long-term reliability have been obtained. Duplex dielectric capacitors were prepared on glass substrates by anodization of sputtered beta tantalum electrodes and vacuum sublimation of bulk silicon monoxide. Variables studied were silicon oxide thickness and gold versus nichrome-gold counterelectrodes. Comparative evaluations were made by leakage current tests and life tests at several voltages and temperatures. A 75-volt dc leakage current criterion indicates that high initial capacitor yields of 98-100 percent can be obtained with 130-volt anodic Ta 2 O 5 , either 2800 .A or 5000 A of silicon oxide, and both types of counterelectrode. These capacitors perform reliably at 50 volts and 85°C. Capacitors with gold counterelectrodes and the thicker films of silicon oxide exhibited a lower failure rate at the higher life test voltages and temperatures