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.