Adhesion Microscopy as a Nanoscale Probe for Oxidation and Charge Generation at Metal-Oxide Interfaces

We introduce a method combining adhesion and conductivity measurements using conduction atomic force microscopy (AFM) to infer the localized surface redox reactions and charge generation resulting from defects created during the electrical stressing of a thin oxide film. The method is demonstrated on a 3.3 nm thick SiO2, which is stressed by applying voltage to the AFM tip until soft dielectric breakdown (SBD) occurs, with the localized current-voltage characteristics and the tip-surface adhesion forces measured before and after the SBD event. The results show that under SBD, the field-driven diffusion of oxygen ions to the AFM tip leads to greatly enhanced adhesion because the oxygen reaching the surface forms strong chemical bonds with the tip material via oxidation. The electrical stressing also generates charged oxygen vacancy defects, and these are observed as an enhanced adhesion arising from image charge forces. The data presented can be corroborated to the physics of dielectric breakdown in transistor gate materials and conductive filament formation in memristor devices and could be extended to other technologies involving diffusion and surface reactivity of oxygen, e.g., solid oxide fuel cells and catalytic supports.