Principle set-up for local anodic oxidation:

A DC voltage is applied to the tip with respect to the sample. The resulting electric field forces then the sample to oxidise locally.

Here, the formation of a single (tunneling) barrier within a thin metal film is shown. The tip repeatedly scans along a single line. The formation of the barrier is controled by monitoring the conductance through the device while oxidising. Note the plateau like structures indicating quantised conductance through the barrier. At t=300s the oxide actually forms a completely insulating barrier.

AFM top view image of a Ti/TiOx barrier. The image shows the 70nm wide metallic wire(black), defined by AFM induced oxide, and the barrier. The cross section shows the barrier to be 21nm wide.