Mark Viste

Understanding internal stress evolution mechanisms associated with field crystallization of anodic tantalum oxide

Previous study has demonstrated that field crystallization of anodic tantalum oxide (ATO) films is associated with large in-plane compressive stresses that evolve in several characteristic stages. The mechanism(s) associated with this stress evolution are not well defined, and the impact of these stresses on the field crystallization process requires further understanding. This work reports investigations of the stress evolution that are designed to evaluate contributions from hydration of the amorphous Ta oxide (ATO), electrostriction and crystal growth. Stresses were monitored in situ with a multi-beam optical technique, to elucidate these different contributions. Measurements were conducted during exposure to the acid electrolyte with and without an applied field, and also during different growth stages. The stress evolution mechanisms include contributions from reactions with the electrolyte (hydration), electrostriction and growth of amorphous and crystalline oxides. In addition, a strong linear correlation between stress evolution and crystal growth was established during longer exposure to electric field.

Reassessment of degradation mechanisms in anodic tantalum oxide capacitors under high electric fields

High-voltage Ta capacitors have broad applications in various electric systems. Anodically grown, amorphous tantalum oxide (ATO) serves as the dielectric in these capacitors. A detailed understanding of the behavior of ATO exposed to high electric fields is highly desirable and is reflected in a number of prior investigations into this subject. In the conventional view, the electric field promotes the growth of crystalline oxide, which is electrically more conductive and thus leads to the degradation of the dielectric. This interpretation is re-examined using several advanced characterization techniques. The results indicate that oxidation of the ATO and underlying Ta occurs preferentially at specific locations. Subsequent crystallization at these sites was only observed in specimens where this enhanced oxidation led to cracking in the ATO. The results with different Ta materials indicate that these cracks are more prevalent with rougher surfaces. The reported sequence of mechanisms that leads to crystallization provides new insight that can potentially be employed to improve the reliability and stability of Ta capacitors.